M6: addr_mode column on xrefs + extended store/load classes

Adds finer-grained addressing-mode classification to every data xref row
plus new dispatch for instruction families not previously emitted:
- New `xrefs.addr_mode VARCHAR NULL` column. NULL for control-flow edges
  (call / ind_call / j / br); one of d_form / lis_addi / lis_ori /
  multiword / x_form_indexed / x_form_byterev / atomic / dcbz for data
  edges. Index idx_xrefs_addr_mode.
- New `xenia_analysis::xref::AddrMode` enum + Xref::addr_mode field.
- Opcode 46/47 (lmw/stmw) expand to one xref per slot — D-form multi-word
  load/store now resolves all (32-rS) consecutive addresses.
- Opcode 31 X-form dispatch — stwx/stbx/sthx/stwux/stbux/sthux/stdx/stdux,
  lwzx/lbzx/lhzx/lhax/lwzux/lbzux/lhzux/lhaux/ldx/ldux,
  stwcx./stdcx. (atomic),
  stwbrx/sthbrx/lwbrx/lhbrx (byte-reverse),
  dcbz (cache-line clear).
- X-form rows are emitted ONLY when both rA and rB resolve to known
  constants (rare but present); the dominant runtime-indexed pattern
  remains correctly skipped.

Sylpheed yield (regen on master + merge):
- 442 newly-detected x_form_indexed reads (lwzx/lhzx into static tables).
- 40 newly-detected atomic writes (stwcx./stdcx. with resolvable address).
- 28,834 lis_addi refs, 18,485 d_form reads, 3,288 d_form writes — every
  pre-existing data row now tagged.
- 0 multiword / dcbz / byterev (these instructions exist but aren't on
  lis+addi-tracked code paths).

Tests 633→636 (+3 xref unit tests covering AddrMode tag uniqueness,
data-edge addr_mode round-trip, control-edge None invariant). Schema
golden updated (xrefs gains addr_mode column).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/xenia-analysis/tests/db_schema_golden.rs

@@ -212,6 +212,7 @@ fn db_schema_matches_expected_columns() {
             ("source", "BIGINT"),
             ("target", "BIGINT"),
             ("kind", "VARCHAR"),
+            ("addr_mode", "VARCHAR"),
             ("instruction", "VARCHAR"),
             ("source_func", "BIGINT"),
             ("source_label", "VARCHAR"),