feat: receipt unlink and attachment withdrawal

Posted entries stay immutable (R-3), but their evidence can now change the
same way the ledger does: by appending events, never by editing or
deleting. New R-13a.

- books.unlink_receipt {attachment_id, reason} journals receipt.unlinked
  and leaves the receipt filed, unlinked. The receipt policy holds: an
  entry that requires a receipt can't lose its last one (structured
  receipt_required error), so a replacement is attach-new, then
  unlink-old. Drafts and reversals are exempt, as at posting.
- books.withdraw_attachment {attachment_id, reason} journals
  attachment.withdrawn for an unlinked receipt or a filed document: hidden
  from list_receipts (unless include_withdrawn), never linkable again. The
  blob stays in the store, hash-verified and readable.
- link_receipt now refuses an already-linked or withdrawn attachment.
  Before, re-linking silently moved a receipt off a posted entry,
  bypassing the policy; evidence now leaves an entry only through a
  recorded unlink. dispose_asset checks its receipt ids before posting
  instead of after.
- Schema v5 adds attachments.withdrawn_at/withdrawn_reason; existing
  stores migrate on open and replay rebuilds the projection.

Tests: store test (errors, reopen, rebuild, verify, journal history) and
an MCP stdio test (policy refusal, replace flow, withdraw, listings,
under-threshold unlink). Dry-run and live use on the the first company books:
8 migrated .eml attachments swapped for their PDF renders, trial balance
unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MtYEaR6M6Ybdhm6X98BWNu

crates/ledger-store/src/schema.rs

@@ -5,7 +5,7 @@
 //! is the current version.
 
 /// Current schema version.
-pub const SCHEMA_VERSION: u32 = 4;
+pub const SCHEMA_VERSION: u32 = 5;
 
 /// Migrations, in order. `MIGRATIONS[i]` migrates from version `i` to `i+1`.
 pub const MIGRATIONS: &[&str] = &[
@@ -172,5 +172,12 @@ CREATE TABLE IF NOT EXISTS payroll_lines(
 );
 CREATE INDEX IF NOT EXISTS idx_payroll_lines_run ON payroll_lines(run_id);
 ALTER TABLE lines ADD COLUMN class_id INTEGER;
+"#,
+    r#"
+-- v5: withdrawn attachments. Withdrawal retires a receipt or document from
+-- listings and linking; the bytes stay in the blob store and the journal
+-- keeps the filing and the withdrawal.
+ALTER TABLE attachments ADD COLUMN withdrawn_at TEXT;
+ALTER TABLE attachments ADD COLUMN withdrawn_reason TEXT;
 "#,
 ];