fix: restore text-select in the Markdown preview (re-vendor client v0.16.3)

[adnaan]

The bug

Selecting a phrase in the rendered-Markdown Preview view no longer showed the floating Comment button, so you couldn't comment on rendered text — only the raw/diff view worked. Reported while testing the #110 preview work; it is not caused by #110 (it reproduces on main).

Root cause

The block-surface text-select logic — the data-surface="block" branch in the client's textOffsetsFromSelection, which resolves a native selection to the touched [data-from]/[data-to] blocks' source line range — lived on an un-merged client branch. It was vendored in when rendered-Markdown comments shipped (#84), then silently lost on a later re-vendor (the v0.16.x client bump for read-progress, #128) from a client main that never had it. Without it, the directive fell through to the code-surface path, looked for [data-line] (which .md-rendered blocks don't have), returned null, and hid the button.

Evidence: the #84-era vendored bundle had 4 data-surface references; the current one had 2.

Fix

• The block-surface support was merged into the client and released as v0.16.3 (= v0.16.2 + that one commit — no other client changes). This PR re-vendors it via make sync-client.
• The only file delta is the client JS bundle (CSS was byte-identical).

Guard (why this slipped)

The browser e2e that catches it (TestE2E_TextSelectRenderedMarkdown) does fail on the regression — but browser e2e are skipped in CI, so nothing gated the re-vendor. Added a plain go test (TestVendoredClientHasBlockTextSelect) that asserts the vendored bundle still carries the data-surface="block" branch. It runs in CI and would have caught this exact regression at re-vendor time. (Sentinel verified: present in v0.16.3, absent in the regressed bundle.)

Verification

• TestVendoredClientHasBlockTextSelect — pass (fails against the regressed bundle).
• e2e TestE2E_TextSelectRenderedMarkdown — now passes (was failing on main); full text-select e2e suite green.
• Full non-browser suite green; go build ./... clean.
• The client delta is exactly the block-surface addition, so read-progress / heartbeat / other client behaviour is unchanged.

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