fix(fb-35): address PR #126 round 1 review

- fetch_span: panic-fix on line_start > total / empty doc
  (return empty text + effective_end = line_start - 1 instead of
  out-of-bounds slice)
- truncated: reserved for budget-driven truncation only; line
  range clamp signaled via effective_end < line_end
- spec / SKILL.md / README: align rejection wording to "PDF /
  audio" (matches code; Image OCR allowed for span)
- store: warning comment on list_chunk_ids_for_doc — chunk_id
  hash sort does NOT preserve document position; real fix is a
  chunks.ordinal column, tracked as follow-up
- surrounding_chunks: saturating_add to defend against u32::MAX
  context arg on 32-bit targets
- tests: line_start > total returns empty + chunk context at
  doc boundary clamps lower bound

Deferred nits (follow-up): table-separator strict CommonMark form;
MCP per-mode strict validation; CLI chunk_id truncation in plain
output. None block correctness.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/superpowers/specs/2026-05-09-p9-fb-35-verbatim-fetch-design.md

@@ -14,13 +14,13 @@ date: 2026-05-09
 
 ## Goal
 
-agent 가 search hit / RAG citation 의 `chunk_id` / `doc_id` 로 raw verbatim text 를 deep-link fetch 할 수 있는 surface. CLI 와 MCP 동시 노출. 3가지 mode (chunk / doc / span). PDF / image 의 line-based span 은 명시적 거절 (`error.v1.code = span_not_supported`).
+agent 가 search hit / RAG citation 의 `chunk_id` / `doc_id` 로 raw verbatim text 를 deep-link fetch 할 수 있는 surface. CLI 와 MCP 동시 노출. 3가지 mode (chunk / doc / span). PDF / audio 의 line-based span 은 명시적 거절 (`error.v1.code = span_not_supported`). image OCR text 는 line-addressable 이라 span 허용.
 
 ## Behavior contract
 
 ### Source of truth
 
-모든 text 는 `CanonicalDocument` / `chunks.text` 에서 가져온 정규화된 markdown. 원본 raw bytes (`assets.storage_path` 의 파일) 는 노출 안 함 — 사용자가 필요하면 직접 read. PDF / image 도 동일 surface (page-text / OCR text).
+모든 text 는 `CanonicalDocument` / `chunks.text` 에서 가져온 정규화된 markdown. 원본 raw bytes (`assets.storage_path` 의 파일) 는 노출 안 함 — 사용자가 필요하면 직접 read. PDF / audio / image 도 동일 surface (page-text / transcript / OCR text). 단 line-based span 은 PDF / audio 거절 — image OCR 은 line-addressable 이라 허용.
 
 ### CLI subcommand