Unit-test the eight trap effects against the C reference (closes #14)

`trapHandlers` had eight entries and zero direct tests, on the one
subsystem besides combat that can kill the hero outright. New
`game/traps_test.go` covers all eight arms of `move.c be_trapped`, the
prologue every trap runs through, and the `rust_armor` tail `T_RUST`
calls. Test-only: no game code changes.

Every expected value is transcribed from `origin/c-master` (`move.c`,
`misc.c`, `fight.c`, `monsters.c`, `rogue.h`) and quoted in the file. No
divergence from C was found.

The trap set is `rogue.h` 192-200: there is no separate "poison dart"
kind — `T_DART` is the poisoned dart — and `T_MYST`, the eleven-way
`rnd(11)` message switch, is the eighth. Details the tests are built
around: `BEARTIME`/`SLEEPTIME` are `spread(3)`/`spread(5)`, both of which
reduce to `rnd(0)` and so cost no random number, which is asserted as
well as their values; `T_ARROW` swings at `s_lvl - 1` and `T_DART` at
`s_lvl + 1`; and the strength loss is gated on `!ISWEARING(R_SUSTSTR) &&
!save(VS_POISON)`, whose short circuit means the ring saves a random draw
as well as the strength.

Damage dice and swing arguments are checked by sweeps rather than single
shots: `rnd(n)` is "raw value % n", so one draw cannot separate a d6 from
a d5, and a forced hit or miss cannot see a wrong `at_lvl`. Both shapes
were forced by mutation runs that the single-shot versions survived.

`be_trapped` takes a coordinate, and which coordinate decides whether
`T_TELEP`'s `mvaddch(tc, TRAP)` does anything. Sprung under the hero
(`move.go` 105-108, `case Floor`) the line is redundant: `tc` is the
hero's square, already stamped `TRAP` by the prologue and redrawn by
`teleport()`'s opening `mvaddch(hero, floor_at())`. Walked onto
(`move.go` 94-98, `case Trap`) it is the only writer: `tc` is the square
being stepped onto while the hero still stands on the previous one,
`leave_room` writes blanks and never `TRAP`, and the arm returns before
`finishMove` so no `look()` follows. Both shapes are tested.

The two death messages are deliberately uncovered: each is printed
immediately before `death()`, which reaches `myExit` and `os.Exit`, so
provoking either would kill the test binary. The hero is pinned with
`fortify()` and the damage is checked by replaying C's arithmetic. They
are the only two: `rust_armor`'s `|| ISWEARING(R_SUSTARM)` operand and
its `if (!to_death)` message suppression are covered as well.

TODO.md

@@ -70,6 +70,92 @@ of Completed Steps.
   reach only mean something against known walls and a known passage number. All
   27 mutations tried against the new tests were caught.
 
+- 2026-08-09 Trap unit-test coverage (`test/traps-coverage`, closes #14):
+  `trapHandlers` had eight entries and **zero** direct tests, on the one
+  subsystem besides combat that can kill the hero outright. New
+  `game/traps_test.go` (19 tests, 15 subtests) covers all eight arms of
+  `move.c be_trapped`, the prologue every trap runs through, and the
+  `rust_armor` tail `T_RUST` calls. Package coverage 56.2% -> 57.9% measured on
+  `main` at `bf820e3`, the branch point, before the sticks tests landed. Every
+  expected value is transcribed from `origin/c-master` and quoted in the file.
+  **No divergence from C was found.**
+
+    The issue body's trap list was wrong and the correction is the first thing
+    worth recording: there is no separate "poison dart" trap — `T_DART` **is**
+    the poisoned dart, its death message being "a poisoned dart killed you" —
+    and the list omitted `T_MYST`, the mystery trap, whose arm is an eleven-way
+    `rnd(11)` message switch. `rogue.h` 192-200 is the authority
+    (`T_DOOR`/`T_ARROW`/`T_SLEEP`/`T_BEAR`/`T_TELEP`/`T_DART`/`T_RUST`/`T_MYST`,
+    `NTRAPS` 8) and the Go `TrapKind` iota matches it index-for-index.
+
+    Three C details the tests are built around. (1) `BEARTIME` and `SLEEPTIME`
+    are `spread(3)` and `spread(5)` (`rogue.h` 108-109), and `spread` is
+    `nm - nm/20 + rnd(nm/10)`; for both, `nm/10` is 0 and C's `rnd` short
+    circuits a zero range without touching the generator, so each is an exact
+    constant that costs **no** random number — and the tests assert the no-draw
+    half as well as the value, because a stray draw desynchronises the
+    seed-compatible stream. (2) `T_ARROW` swings at `s_lvl - 1` and `T_DART` at
+    `s_lvl + 1`: opposite signs, which is exactly the kind of detail a
+    transliterating port drops. (3) The strength loss is gated on
+    `!ISWEARING(R_SUSTSTR) && !save(VS_POISON)`, and the `&&` is load-bearing —
+    with the ring on, C never rolls the save, so the arm must spend two random
+    numbers and not three.
+
+    Two shapes worth keeping, both forced by mutation results rather than
+    foresight. Damage dice are checked by a **sweep**, not one shot: `rnd(n)` is
+    "raw value % n", so a single draw agrees between a d6 and a d5 five times in
+    six and leaves the generator identical either way — the first draft's
+    single-trial arrow test passed with `roll(1,6)` mutated to `roll(1,5)`.
+    Likewise the swing arguments are pinned by a 200-trial boundary sweep at a
+    mid-range to-hit target: a forced hit and a forced miss cannot see a wrong
+    `at_lvl` or a dropped `op_arm`, because both arms are reachable at any level
+    and swing spends one `rnd(20)` regardless.
+
+    Mutation-proved, 33 mutations, each reverted, and every one of them is now
+    caught. Three were **not** caught on the first pass and the tests were
+    strengthened until they were, which is the useful part of the record. (a)
+    Deleting `new_level()` from `T_DOOR` left the suite green: `be_trapped`'s
+    own prologue stamps the trap glyph into the cell the hero fell through, so
+    "the map changed" is true even with no new level dug. The test now counts
+    differing cells — exactly one can change that way — and also requires the
+    staircase to move and the hero to be re-placed. (b) The `roll(1,6)` case
+    above.
+
+    (c) **`be_trapped` takes a coordinate, and which coordinate decides whether
+    `T_TELEP`'s `mvaddch(tc, TRAP)` does anything.** Deleting that line first
+    left the suite green, and the first draft wrote that off as an unavoidable
+    redundancy — wrongly, because the test only exercised one of the two call
+    sites. `move.go` 105-108 (`case Floor`) springs a trap under the hero and
+    passes `p.Pos`; there `tc` **is** the hero's square, the prologue has
+    already set its `p_ch` to `TRAP`, and `teleport()` opens by drawing
+    `floor_at()` — which returns `chat(hero)` — over it, so the glyph is on
+    screen before the line runs. But `move.go` 94-98 (`case Trap`), the ordinary
+    walk onto a hidden trap, passes `nh`, the square being stepped **onto**,
+    with the hero still on the previous square: `teleport()`'s opening `mvaddch`
+    paints the old square, `leave_room` writes blanks and never `TRAP`, and
+    nothing calls `look()` afterwards because the `case Trap` arm returns before
+    `finishMove` for a teleporter. There `mvaddch(tc, TRAP)` is the only writer,
+    exactly as C's comment says.
+    `TestTrapTeleportDrawsTheTrapOnTheSquareSteppedOnto` springs the trap at a
+    floor square next to the hero and pins it: unmutated the screen at `tc`
+    reads `^`, with the line deleted it reads `.`.
+
+    The other 30 each failed their own test and only their own; two also moved
+    `TestAutoSaveOnSignalRacesTurnLoop`, which drives real turns and is
+    legitimately sensitive to `BEARTIME` and to armor rusting.
+
+    Deliberately uncovered: the two death messages, "an arrow killed you" and "a
+    poisoned dart killed you". Each is printed immediately before `death()`,
+    which reaches `myExit` and `os.Exit`, so provoking either would take the
+    test binary with it; the hero is pinned with `fortify()` and the damage
+    rolls are checked by replaying C's arithmetic instead of by letting HP reach
+    zero. They are the only two: `rust_armor`'s `|| ISWEARING(R_SUSTARM)`
+    operand and its `if (!to_death)` suppression of the rust-vanishes message,
+    the last predicates that had no assertion, are pinned by
+    `TestTrapRustHonoursTheRingAndTheToDeathFlag`. This entry does **not**
+    rotate `Next Step`: #14 was an out-of-band gap found while surveying, not
+    part of the rings/sticks/wizard step.
+
 - 2026-08-09 Ring unit-test coverage (`test/rings-coverage`, closes #5): the
   first third of the standing coverage step. `game/rings.go` had **zero** tests
   — not one of the 32 in the suite touched wear, removal, hand choice, or the