fix: unblock session writes by skipping full-state validation

[zxch3n]

Related issue

Independent availability hotfix; the HistoryWriter work remains deferred in PR #460.

Problem / pressure

Full-state Mirror validation rejects valid local sends when unrelated synchronized history contains an unknown item type or a malformed known item. The writer refactor has broader fork and rollback interactions and should not be an urgent prerequisite.

Summary

Set validateUpdates: false at the two existing session Mirror construction sites (renderer and CLI). No new writer, schema, dependency, storage, copy, rollback, or transport changes. Other Mirrors and explicit external-input parsers retain their existing checks.

Add real-Loro regressions and a production construction-site check. Record the temporary trade-off in a bilingual Agent Note and draft Spec.

Before / after

Before	After
Old incompatible history can veto an unrelated valid send.	Session Mirror skips whole-state validation; the tested append and text-update paths continue.
Mirror provides an extra runtime check for new local values.	That safety net is temporarily absent; this is NOT proof that malformed local writes are impossible.

Test plan

• Real-Loro enabled/disabled control, live import and snapshot reopening, append and text updates, unchanged old history/container identity, concurrent future-item updates, and unchanged external parser rejection.
• Shared suite: 92 files / 1037 tests passed; shared typecheck passed.
• Targeted Prettier, Oxlint (0 errors; existing warnings), docs check, and git diff check passed.
• Reused local Loro 1.15.1 / Mirror 2.3.1 dependencies. Full pnpm check stopped at missing dependencies in other packages of this isolated worktree; clean-install CI remains required.
• No deployed desktop/mobile acceptance, disk-failure recovery, or 3000-turn performance acceptance. No merge or release performed.

Context handoff

Instructions for reviewing agents

• Review focus: Confirm only the two session construction sites bypass validation and existing external parsers are untouched.
• Decisions to challenge: This explicitly trades a runtime safety net for immediate availability; do not mistake it for changed-input validation.
• Plausible failures / evidence gaps: Readers may still reject incompatible items, old unpatched clients remain affected, and new malformed local values are no longer caught by Mirror.

Authoring context

• User goal / directives: Split a minimal hotfix from the larger writer PR and defer the latter for fixes and multi-round independent review.
• Constraints / non-goals: No HistoryWriter extraction, schema widening, migration, dependency change, merge, or release.
• Risk-bearing decisions: Disable full-state session update validation while retaining explicit input parsers; there is no new per-field safety boundary in this PR.
• Destructive or irreversible behavior: No new deletion or rewrite operation; existing initialization and persistence behavior stays unchanged.
• Deliberately not done or tested: Deployed-client acceptance and the deferred writer review are separate work; full local checks are blocked by isolated-worktree dependency setup.
• Unknowns / confidence: Real-replica checks establish the tested local write paths, not arbitrary input safety or end-to-end delivery; CI is required before merge consideration.