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/src/db.rs

@@ -390,7 +390,8 @@ impl DbWriter {
             CREATE TABLE xrefs (
                 source       BIGINT NOT NULL,  -- VA of the referencing instruction
                 target       BIGINT NOT NULL,  -- VA of the referenced destination
-                kind         VARCHAR NOT NULL, -- call | jump | branch | data_read | data_write | data_ref
+                kind         VARCHAR NOT NULL, -- call | ind_call | j | br | read | write | ref
+                addr_mode    VARCHAR,          -- M6 sub-classification of how source computes target (NULL for control-flow)
                 instruction  VARCHAR,          -- mnemonic of source instruction; NULL if not in binary
                 source_func  BIGINT,           -- VA of the function containing source; NULL if unknown
                 source_label VARCHAR,          -- label at source; NULL if none
@@ -418,6 +419,7 @@ impl DbWriter {
             ("idx_methods_function",          "CREATE INDEX idx_methods_function          ON methods(function_address)"),
             ("idx_classes_rtti",              "CREATE INDEX idx_classes_rtti              ON classes(rtti_present)"),
             ("idx_strings_encoding",          "CREATE INDEX idx_strings_encoding          ON strings(encoding)"),
+            ("idx_xrefs_addr_mode",           "CREATE INDEX idx_xrefs_addr_mode           ON xrefs(addr_mode)"),
             ("idx_xrefs_target",              "CREATE INDEX idx_xrefs_target              ON xrefs(target)"),
             ("idx_xrefs_source",              "CREATE INDEX idx_xrefs_source              ON xrefs(source)"),
             ("idx_xrefs_source_func",         "CREATE INDEX idx_xrefs_source_func         ON xrefs(source_func)"),
@@ -1062,10 +1064,12 @@ fn insert_xrefs_streaming(
                 xref.source, func_analysis, labels,
             );
 
+            let addr_mode = xref.addr_mode.map(|m| m.tag());
             appender.append_row(params![
                 xref.source as i64,
                 target as i64,
                 kind,
+                addr_mode,
                 instruction.as_deref(),
                 source_func,
                 source_label.as_str(),