Emit slog attributes from every handler (closes #19, closes #24)

Handle never read the record's attributes, and WithAttrs and WithGroup
returned the receiver unchanged. The JSON and webhook handlers also marshaled
slog.Record itself, whose attributes are unexported.

Each handler now carries a handlerAttrs value, copied rather than mutated, so
sibling loggers cannot leak attributes into each other. JSON and webhook
output nest groups as objects; the console appends key=value pairs with dotted
group keys, quoted as slog.NewTextHandler quotes them, invalid UTF-8
included. Logged values are never written to. In JSON the record's own fields
win a key collision, and a repeated key keeps its last value unless both are
groups, which merge; the README documents both.

Deviation: DEL is quoted on the console; the stdlib leaves it bare.

Model: opus-5-5

TODO.md

@@ -24,6 +24,11 @@ files it depends on: .golangci.yml, REPO_POLICIES.md, .editorconfig,
 
 # Completed Steps
 
+* 2026-08-10: fixed every handler discarding slog attributes: console,
+    JSON and webhook handlers now emit record attributes, accumulate
+    WithAttrs without mutating the receiver, and honour WithGroup;
+    slog.Group values nest and LogValuer values are resolved; console
+    keys and values holding invalid UTF-8 are quoted
 * 2026-08-07: added canonical `.golangci.yml` (v2 schema), pinned the
     `Dockerfile` lint stage to golangci-lint v2.12.2 (tag+digest), and
     fixed all findings the v1→v2 jump surfaced without changing any
@@ -50,6 +55,8 @@ files it depends on: .golangci.yml, REPO_POLICIES.md, .editorconfig,
     the working tree
 * Pick one tag scheme before the next release (v1.0.0 vs 1.0.1 are
     inconsistent)
+* Tag v1.0.2, with the leading v, once the attribute fix lands, so
+    consuming repos can move off pseudo-version pins in one step
 * Fix RELP output to cache (from old TODO)
 * Re-add RELP delivery over TCP to remote rsyslog imrelp; removed
     2024-06-14 because it did not build (README planned feature)