docs: state verify-build's dist/ guarantee at the width it enforces (closes #331)

check_dist_tree walks -type f -o -type l, so the emitted-tree cross-check covers
regular files and symlinks under dist/. README.md said "nothing under dist/ that
the build did not write", which is broader: fifos, sockets, device nodes and
empty directories are not checked.

The exclusion stays. A build emits none of those types, none can carry a
shippable payload, and grep on a fifo would hang rather than fail. README.md,
the script's header comment and the check_dist_tree comment now say so in the
same words, so the code and the docs cannot drift apart again.

Documentation only: no non-comment line of script/verify-build changed.

README.md

@@ -81,17 +81,21 @@ lives.
 one of the bundles containing `src/shared/constants.js` — into a build receipt,
 and `script/verify-build` checks `dist/` against that receipt: every recorded
 file present with exactly the recorded bytes, every audited bundle carrying the
-requested `DEBUG` marker, and nothing under `dist/` that the build did not
-write. The `Makefile` creates the receipt path with `mktemp` per invocation,
-outside the repo, and deletes it afterwards.
+requested `DEBUG` marker, and no regular file or symlink under `dist/` that the
+build did not write. The `Makefile` creates the receipt path with `mktemp` per
+invocation, outside the repo, and deletes it afterwards.
 
 That is what ties the check to a build rather than to a directory. What it
 establishes is narrow and worth stating exactly: `dist/` is byte for byte the
-output of the `build.js` run that just finished, with nothing added, removed or
-altered in between. It establishes nothing about whether the source tree or
-`build.js` were honest, and it offers nothing to someone handed a `dist/` from
-elsewhere — without the receipt from its own build there is no input to the
-check. Verifiable provenance for a third party is signing, which this is not.
+output of the `build.js` run that just finished, with no regular file or symlink
+added, removed or altered in between. Regular files and symlinks are the whole
+of what the tree walk covers; fifos, sockets, device nodes and empty directories
+under `dist/` are not checked, because a build emits none of them, none can
+carry a shippable payload, and `grep` on a fifo would hang rather than fail. It
+establishes nothing about whether the source tree or `build.js` were honest, and
+it offers nothing to someone handed a `dist/` from elsewhere — without the
+receipt from its own build there is no input to the check. Verifiable provenance
+for a third party is signing, which this is not.
 
 There is deliberately no target that re-verifies an existing `dist/` on its own.
 The list of files to check has to come from the build that produced them; read
@@ -143,12 +147,13 @@ provide:
   serves. Run deliberately, never as part of a build: the output is committed
   and there is no runtime fetch, so the shipped list is as fresh as the last
   vendoring run that was released
-- `script/verify-build --expect release|debug --receipt PATH` — assert that
-  `dist/` is exactly what the build that just ran emitted, and that the compiled
-  `DEBUG` state of the bundles in it is the one that was asked for. Both
-  arguments are required and neither has a default: the expected mode is stated
-  by the caller rather than read from `AUTISTMASK_DEBUG`, and the file list
-  comes from the build's receipt rather than from `dist/` (see
+- `script/verify-build --expect release|debug --receipt PATH` — assert that the
+  regular files and symlinks under `dist/` are exactly what the build that just
+  ran emitted (other file types are out of scope), and that the compiled `DEBUG`
+  state of the bundles in it is the one that was asked for. Both arguments are
+  required and neither has a default: the expected mode is stated by the caller
+  rather than read from `AUTISTMASK_DEBUG`, and the file list comes from the
+  build's receipt rather than from `dist/` (see
   [Build Receipts](#build-receipts)). Run automatically at the end of
   `make build` and `make build-debug`; fails loudly rather than passing whenever
   it cannot determine something. Not part of `make check`, which does not depend