A native Interchange mailbox, and nothing else

[TheGreatAxios]

Summary

Owner ruling: @corbits/mailbox exists ONLY to give human principals a
native Interchange mailbox (createPrincipalMailboxStore /
NativeMailboxStore over mailbox.principal_mail + mailbox.mailbox_state,
vendored @intx/mailbox on top). Everything else — triage
(priority/classification/status/assignee), delegation/assign, this
package's own threading/search, /me/threads*, the pre-native
read/write model — is deleted.

Four commits, in order:

1. Delete tests of the pre-native mailbox model — every test
   file/describe exercising triage columns, readAt/archivedAt/trashedAt,
   this library's own threading/search, /me/threads*, and the mount
   routes/features tied to them.
2. Routes over the native store — mount.ts is now a thin wrapper:
   GET /me/inbox (vendored executeSearch over open(folder), keyset
   by uid, envelope+raw fetched per ref), POST /me/inbox/:uid/read| unread (addFlags/removeFlags \Seen), archive|trash|restore
   (moveNativeMailboxMessage), GET /me/inbox/events (SSE, unchanged).
   read.ts, thread.ts, mutations.ts, vocabulary.ts are deleted.
   write.ts (writeMailboxMessage, deliverInboxItems) and
   persist.ts (the transport dual-write seam) now land every write
   through NativeMailboxStore.append.
3. Drop the pre-native columns — migration 0005_drop_pre_native_ columns drops mailbox.mailbox (the read_at/archived_at/trashed_at/
   priority/classification/status/assignee management table — nothing
   reads it anymore) and sets principal_mail.uid/modseq NOT NULL.
   schema.ts, schema-check.ts, and the DDL parity test follow.
4. Docs — README rewritten to the one-job statement and the
   surviving routes/write paths; package.json bumped to 1.0.0 for
   the breaking cut.

Line counts

• origin/main (pre-vendoring, full pre-native model): 15,632 lines in src/
• Branch base (54f657f, native store added, old model still present): 16,288 lines
• This PR's src/ after Slice B: 6,355 lines — a 60% cut from the branch base

Vendored ledger

Unchanged by this slice — VENDORED.md still lists vendor/intx-mailbox,
vendor/intx-mime, vendor/intx-types at the same pinned commit, kill
condition still "first npm publish of @intx/mailbox". Note for whoever
picks this up next: publish @intx/mailbox (and a matching
@intx/mime/@intx/types npm refresh) to retire all three vendored rows.

Remaining public API

• mountMailbox (routes above), MAX_MAILBOX_PAGE_LIMIT, MAX_PENDING_SSE_EVENTS
• runMailboxMigrations, MigrationChecksumError
• assertExpectedColumnTypes, expectedColumnTypes, SchemaTypeMismatchError
• createMailboxDb
• createPrincipalMailboxStore, openNativeMailboxStore, moveNativeMailboxMessage
• principalMail, mailboxPgSchema (schema/table exports)
• assertMailboxScope, assertMailboxTenantId, MAX_MAILBOX_FRAME_BYTES, assertMailboxFrameBytes
• purgeTenantMailbox, purgePrincipalMailbox
• createInMemoryMailboxEventBus, MailboxEventSchema, MAILBOX_EVENT_OPS (now 6 ops, no enrich/assign)
• writeMailboxMessage, deliverInboxItems
• buildMailFrame, generateMailboxMessageId, MESSAGE_ID_FALLBACK_DOMAIN
• createMailboxPersist, MAX_MAILBOX_RECIPIENTS
• parseAddressList, resolveMailboxRecipients

Deleted from the public API: writeMailboxMessages, mailboxKey,
MAX_MAILBOX_REFS/refs support, all of read.ts's exports (list/
filter/sort/enrichment/sender-display), all of thread.ts's exports,
all of mutations.ts's exports (enrich/assign/bulk), all of
vocabulary.ts's exports — mountMailbox no longer takes a
vocabulary or resolveSenderDisplays option.

Test plan

• bun run typecheck green (pre-existing, unrelated errors only in
  examples/reference-host, not touched by this PR)
• bun test src green — 156 pass, 2 pre-existing/unrelated fails in
  recipients.test.ts (isRunAddress from intx-types-compat.ts
  not recognizing ins_ addresses — present before this PR's first
  commit, untouched by any of these changes)
• CI