docs(ui): route the undefined session-id case to the direct seam (RIG-2874)

[rigel-mintaka]

This PR is part of a stack containing 4 PRs:

1. main
2. feat(ui): export a composeBoot seam so the analytics/clients boot order is testable (RIG-2874) #1096
3. "docs(ui): route the undefined session-id case to the direct seam (RIG-2874)" (this PR)
4. fix(ui): trim the boot comments after the composeBoot extraction (RIG-2874) #1104
5. docs(ui): anchor the captureRequest citation by its thrown string (RIG-2874) #1105

Fixes a defect in the frozen design record for the outbound session header.
Matt approved the fix directly. Docs-only; no code changes.

The defect

The record's test-case list assigned the getter returns undefined case to
the capture seam:

- getter returns `undefined` ⇒ header **absent** (not empty) — capture seam.

That placement is unsafe. captureRequest's capturing fetch always
throws (packages/compass-client/src/index.test.ts:93), so the helper gates
on await expect(run(fetch)).rejects.toThrow() (:95). That gate is green
whether the interceptor threw early or reached next normally — it cannot
distinguish the two. Since the guard can throw on undefined, routing this
case onto the capture seam makes the test green on exactly the defect the case
exists to catch.

The record already states this rule correctly for the non-Latin-1 case
(including "Do not 'simplify' this case back onto captureRequest") and
already requires the triple assertion, where assertion 3 — the awaited result
is the sentinel next produced — is what proves nothing threw. Line 582
simply contradicted the record's own rule.

The fix

The undefined case now reads direct seam, all three assertions above,
with a short justification in the record's existing voice.

The shipped implementation was already correct — #1055 did not follow the
record here, precisely because the prescription was unsafe. Only the record
needed fixing, so there is no behaviour change and no test change.

Verification

The load-bearing claim was checked at source rather than assumed: the
unconditional throw is at index.test.ts:93 and the rejects.toThrow()
gate at :95.

• bun tools/design-ledger-gate/index.ts — OK, 311 ledger rows, 132 record
  headers valid (rc=0)
• moon run root:markdownlint — rc=0

Diff touches one file, 7 insertions / 1 deletion.

RIG-2874

[linear-code]

RIG-2874

[github-actions]

Compass engineering docs preview: https://compass-ui-rig-2874-record-d.compass-eng-docs.pages.dev

Deployed from compass-ui/rig-2874-record-direct-seam at 7651fc1.

Changed pages:

• docs/designs/ui/compass-outbound-session-header/design.md

[trunk-io]

This pull request was merged into main as part of stacked PR 1105.