…rs (MUL-4922)

Phase 1 of the RichContent convergence: collapse the duplicated security
base shared by the two product-level Markdown chains.

Chat (packages/ui/markdown/Markdown.tsx) and Issue/Comment
(packages/views/editor/readonly-content.tsx) each carried a verbatim fork
of the rehype-sanitize schema and urlTransform, and the forks had already
drifted: readonly whitelisted <mark> for `==highlight==`, chat did not.
A security-relevant allow-list maintained in two places means every future
XSS fix has to land twice, and missing one is a hole — this is the hardest
reason for the sweep, ahead of the user-visible feature drift.

- Extract markdownSanitizeSchema + markdownUrlTransform into
  packages/ui/markdown/sanitize.ts and export from the package index.
  Both chains now import the single copy; no local forks remain.
- The canonical schema is the union, so chat gains the <mark> tag name.
  This is the one intentional behavior delta: <mark> is inert and admits
  no attributes, and chat needs it anyway once ==highlight== converges.
- Annotate the schema as rehype-sanitize's Options: exporting it makes the
  previously-inferred hast-util-sanitize type unnameable across packages.

Adds a cross-surface contract test that runs one set of security fixtures
(script, event handlers, javascript: href, data:image vs data:text/html,
mark, slash://) through BOTH surfaces and asserts identical outcomes —
the mechanism that stops a third fork from growing back.

Code-block rendering is deliberately not asserted cross-surface yet: chat
highlights with Shiki, readonly with lowlight, so emitted class tokens
still differ. Converging them is the RichCodeBlock phase and needs the
highlight-engine decision first; only the schema-level allow-list is
shared here.

Verified: pnpm typecheck (6 workspaces), pnpm lint (0 errors),
pnpm vitest run in packages/views (228 files, 2665 tests, all passing).

Co-authored-by: multica-agent <github@multica.ai>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

… blocks (MUL-4922)

Two review follow-ups.

1. Dangling package export
`packages/views` still exported "./common/markdown" after the file was
deleted in the RichContent convergence. TypeScript resolves against the
source tree so typecheck and unit tests both passed; the subpath would only
fail when a consuming app bundled it. No consumer imports it, so this was
latent rather than broken in practice.

Removes the entry and adds packages/views/rich-content/package-exports.test.ts,
which walks every workspace package.json and asserts each export target
exists (wildcards checked by directory prefix). Scoped repo-wide because the
failure mode belongs to package.json, not to this package. Verified the guard
actually fails by re-adding the dangling entry before committing.

2. Near-viewport lazy shell for Mermaid / HTML
Implements the deferred performance contract rather than requesting an
exemption. LazyRichBlock defers each rich leaf until it is within 800px of
the viewport, then latches: once mounted it is never unmounted, so scrolling
past does not re-run Mermaid, rebuild the sandboxed iframe, or discard the
viewer's pan/zoom state.

The stable-size requirement is handled by reserving the block's expected
height before AND after mount, so a block never measures 0px off-screen and
then jumps — the churn that makes a virtualized list mis-estimate item sizes.
The reservation is not a local guess: reservedMermaidHeightPx() reuses the
existing session layout cache (real height on a cache hit, else the documented
280px skeleton) and HTML_BLOCK_PREVIEW_HEIGHT_PX is the pixel twin of the
preview iframe's fixed h-[480px]. Both are exported from the leaves so there
is one source of truth per block type.

Environments without IntersectionObserver (jsdom, SSR) mount eagerly, which is
today's behaviour; rendering nothing would not be.

Verified in real Chromium (jsdom has no IntersectionObserver, so unit tests
only exercise the eager fallback):
- Non-virtualized Issue/Comment with 25 diagrams: 3 mounted, 22 deferred on
  load; after scrolling, mounted grows 3 -> 6 and never drops, confirming the
  latch. This is where the win is real.
- Chat gains less: Virtuoso already caps the DOM to ~4 rows, so only 1 of 4
  shells was deferred. Stated rather than overclaimed.
- Live -> persisted handoff re-checked with the shell in place: scrollTop
  220 -> 220 (delta 0), the tagged diagram DOM node survived, and the run
  reached the persisted state. Contract 1 is not regressed.

Tests: 6 lazy-shell cases (defer, mount-on-approach, latch/no-unmount, equal
reserved height before and after mount, root margin exceeding the chat list's
own overscan, no-IntersectionObserver fallback) plus 3 export-guard cases.

Verified: pnpm typecheck (6 workspaces), pnpm lint (0 errors),
packages/views vitest 242 files / 2752 tests all passing.

Co-authored-by: multica-agent <github@multica.ai>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>