design: TDD 0070 rev 2 — render isolation enforced by the browser

[cahenesy]

/build-tdds run 20261006-220142 halted three times on the same class:
an out-of-repo local file baked into a committed PNG through the
file:// render. Each regex-scanner fix was bypassed by the next gate:

• script navigation;
• <!--> before the doctype;
• unquoted, backtick and NBSP attribute values;
• base64 data:text/css;
• url() in tokens.css or another CSS file.

Rev 2 moves the boundary into the browser:

• render copies docs/ux/ to a private mkdtemp dir outside the repo,
  after a whole-tree symlink check and a re-check of the copy;
• every HTML file starts with plus a CSP meta at byte 0;
• a loopback-only ThreadingHTTPServer serves the copy, with the same CSP
  as a header on every response;
• Chrome loads http://127.0.0.1:/..., with a dead proxy that
  excludes other loopback ports.
  An http page cannot load file: URLs, absolute and ../ paths 404 on a
  server that holds only the copy, and default-src 'self' data: blocks
  everything else. The scan stays as defence in depth.

Also in this revision:

• capture leaves out the proxy flags;
• the scanner rules are written down as built;
• the probe-matrix obs 10b requires the CSS and base64 vectors to pass
  validate and still show 0 canary pixels;
• honest per-file diff estimates with exceptions;
• tests/implement-gate.test.sh is declared;
• a revision record.

Design critique (design-reviewer, fresh worker)

DESIGN_REVIEW: PASS. The isolation was judged sound point by point
(file: from http, translate_path, CSP 'self', the bypass list failing
closed, port binding). All 9 minor findings and nits were folded in
after the PASS:

• the copy lives outside the repo;
• the capture proxy flags are dropped;
• CSP is sent as an HTTP header on every response;
• the symlink scope and scanner rules are written down;
• 10b is strengthened;
• obs 7 now has an observable;
• the FR-94 trace is corrected;
• the revision record is moved;
• the override count is corrected.

Mechanical pre-pass

Structural lint is clean.
PRECHECK_FAIL: tdd-doc-size 612 > 500 is WAIVED via a
## Scope override, chosen by the user. The excess is gate-driven
hardening of an already-built unit, and splitting would orphan the
retained build branch. The reviewer graded the justification
acceptable.

Open assumptions & waivers

• Render isolation mechanism — resolved: the user chose "Revise TDD,
  then Retry" with loopback-HTTP + CSP isolation over patching the
  scanner.
• tdd-doc-size bound — waived: Scope override (rationale above).

After merge: /build-tdds Retry 3 builds rev 2 on the retained branch
build/20261006-220142/0070-ux-record-lib, with no escalation.

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https://claude.ai/code/session_01NpGG21Y2VGGhTQaFr4oshB