v0.26.0: auto-link referenced RFCs in PR text + comments (roadmap #28 Part 1)

Part 1 of item #28: references to existing accepted RFCs inside PR
descriptions and comment text now render as inline links to the
referenced RFC. Parts 2 (offer-to-create) and 3 (offer-to-contribute-
to-pending) are deliberately deferred — Part 1 ships first as the easy
win, per the roadmap row.

Shipped in parallel with the v0.25.0 security-hardening session; this
took the next free version slot (0.26.0) per the roadmap's
"claims the next available version number" rule. Expect a top-of-file
CHANGELOG/VERSION merge with 0.25.0 — distinct concerns, trivial to
resolve.

Backend:
- rfc_links.py (new): builds a term index from the live accepted
  (state='active') RFC corpus and segments plain text into text /
  rfc-link segments. Conservative matching — links only rfc_id tokens
  (RFC-0001), multi-word titles (Open Human Model), and hyphenated
  slugs (open-human-model); a single common-word title/slug is NOT
  linked (would turn every prose "human" into a link). Case-insensitive,
  word-boundary-anchored, longest-match-wins, self-reference suppressed.
- api_prs.py get_pr(): enriches the PR description (description_segments)
  and every PR comment (text_segments).
- api_discussion.py: enriches PR-less discussion comments (text_segments).

Read-time, not submit-time: the roadmap says "at submit time" but the
intent it names is "not as live compose preview", which read-time
honors. Chosen for correctness (links track the live active set —
newly-accepted RFCs start linking, withdrawn ones stop), zero migration,
and cheapness (small cache-resident corpus). Recorded as a §19.3-rule-2
note in the session transcript.

Frontend:
- LinkedText.jsx (new): maps backend segments onto React text nodes +
  anchors. No dangerouslySetInnerHTML — XSS-safe by construction,
  independent of any HTML-sanitization layer. Falls back to raw text
  when segments are absent.
- PRView.jsx: description + PR conversation comment bodies render via
  LinkedText.
- RFCDiscussionPanel.jsx: discussion comment bodies render via LinkedText.
- App.css: .rfc-autolink (subtle accent + dotted underline, tokenized).

Tests: 12 new (test_rfc_links_vertical.py) — 9 scanner units + 3
end-to-end (PR description / review comment / discussion comment all
surface *_segments; self-reference suppression). Full suite 363 green;
frontend builds clean.

No upgrade steps: additive, no migration, no secret, no config.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/api_prs.py

@@ -23,7 +23,7 @@ from typing import Any
 from fastapi import APIRouter, HTTPException, Request
 from pydantic import BaseModel, Field
 
-from . import auth, cache, chat as chat_layer, db, entry as entry_mod, funder, models_resolver
+from . import auth, cache, chat as chat_layer, db, entry as entry_mod, funder, models_resolver, rfc_links
 from .bot import Bot
 from .config import Config
 from .gitea import Gitea, GiteaError
@@ -213,6 +213,12 @@ def make_router(
         path = _file_path_for(rfc)
         head_branch = pr_row["head_branch"]
 
+        # Roadmap #28 Part 1: build the RFC auto-link index once for this
+        # PR view (read-time enrichment against the live accepted-RFC
+        # corpus; see rfc_links.py). exclude_slug suppresses self-links to
+        # this RFC inside its own PR.
+        link_index = rfc_links.build_index(db.conn(), exclude_slug=slug)
+
         # §11.3: PRs are always public; no visibility check.
         main_fetched = await gitea.read_file(owner, repo, path, ref="main")
         main_body = _extract_body(rfc, (main_fetched or ("", ""))[0])
@@ -259,6 +265,13 @@ def make_router(
             for r in msg_rows:
                 messages_by_thread.setdefault(r["thread_id"], []).append(_serialize_message(r))
 
+        # Roadmap #28 Part 1: enrich every comment with RFC auto-link
+        # segments (read-time; see rfc_links.py). The description is
+        # enriched alongside it in the return dict below.
+        for _msgs in messages_by_thread.values():
+            for _m in _msgs:
+                _m["text_segments"] = link_index.segment(_m["text"])
+
         # Per-user seen cursor per §10.3. Anonymous viewers get no
         # cursor — they always see "everything new" but cannot advance
         # the cursor (no row to write to).
@@ -325,6 +338,7 @@ def make_router(
             "pr_number": pr_number,
             "title": pr_row["title"],
             "description": pr_row["description"],
+            "description_segments": link_index.segment(pr_row["description"]),
             "proposed_use_case": _pr_use_case(pr_number),
             "state": pr_row["state"],
             "opened_by": pr_row["opened_by"],