fix(runtime-host): admit structured-only Messages and keep them model-visible

[ggbdpq]

Summary

Fixes #4804. Quote-only and attachment-only turns carried real model-facing context but were rejected twice on the way to the model, both times by a text-only admission assumption:

1. Host admission — decodeMessageAdmissionContent threw Invalid Message text for empty inline text unless skill ids were present, so a staged quote or a lone attachment could not be submitted.
2. Replay visibility — runtimeEventHasModelVisibleContent counted only inline text length, so a persisted user event with empty text and a non-empty quotes array was dropped as blocking-unsupported, producing the durable current-run projection is not replayable / empty_text_skipped failure the issue records.

Now decodeMessageAdmissionContent applies the rule itself after structural decode — empty inline text is admissible when the Message carries quotes or attachments, and a Message with none of the three still throws the same invalid-frame error — and the visibility predicate counts structured user context (quotes/attachments) as model-visible. All turn/message admission call sites share the one function, and the model projection already renders quotes (formatQuoteRefs), so structured-only turns now reach the model with their content.

Verification

Claim	Command	Result
New tests pass with the fix	node --test --test-name-pattern="#4804" on core runtime-event + runtime-host protocol dist tests	2 tests, 2 pass
Red-green	stash the two production files, rebuild, rerun	2 tests, 2 fail; restored → pass
Quote-only admission at the frame level	turn.message.submit with {text:'', quotes:[…]} / {text:'', attachments:[…]}	admitted; {text:''} alone still Invalid Message text
No admission regression	protocol.test.js (78), message-coordinator.test.js (73)	all pass
Format	npm run format:check	exit 0

Known Windows-environment failures encountered while verifying, both reproduced identically with these production files stashed (control runs) and therefore pre-existing: hosted linked child roots share admission… (EBUSY unlink of a temp runtime.sqlite) and one EPIPE shutdown test in host-kernel. The full Runtime Host suite additionally stalls on this machine on an unrelated file; CI on Linux/macOS is the authoritative full-suite run.

Scope note: the renderer-side empty user bubble beside a quote chip is a display projection and is not addressed here; the durable event and the model request are correct after this change.

AI use

Implemented with ZCode (GLM-5.3-Flash): traced the issue's four named boundaries to the two blocking ones, applied the smallest contract change at each, and added red-green tests tagged with the issue number. The commit carries the Generated-by trailer.

Checklist

• Root-cause fix at shared admission/visibility points, not per-surface patches
• Regression tests fail on the old code (verified via stash/rebuild)
• Test style follows each file's existing conventions, issue number in test names
• No new dependencies; renderer display left as a stated follow-up