fix: [ENG-2883] probe only the suffix the agent chose; address PR #690 nits

The warner previously passed if EITHER `<ref>.html` or `<ref>/` existed.
That left a symmetric false-negative class to the dropped ambiguous
case: a `.html` ref pointing at a folder (or a bare ref pointing at a
file) passed silently even though the FE routes by suffix and would
show a dead pill.

Tightened to probe only the form the agent's suffix picked:
- `.html` suffix → check `<ref>.html` (file form) only
- bare → check `<ref>/` (folder form) only

A shape mismatch on disk is no longer ignored — it now surfaces as a
broken-ref warning, with a coaching hint on the bare-form case ("add
.html if you meant a file") since "agent forgot the .html suffix" is
the typical mistake. Updated the JSDoc, tightened the broken-ref
messages to name the missing shape ("no file at …" vs "no folder at
…"), and replaced the two now-impossible "bare ref matches file" /
"`.html` ref matches folder" no-warning tests with their inverses
asserting the new warnings.

Same commit also addresses two cosmetic review nits:
- `renderEnvelope` docblock in brv-curate-tool.ts:266-275 updated to
  describe the head + ⚠ lines shape (was "a single confirmation line").
- Stale sequencing comment in html-writer.test.ts:392 ("before the
  disk write") corrected to "after the atomic write".