v0.5.0

Grows the sdbg headless debugger into a full trace-diff workbench, and adds a
reference/developer-tooling documentation layer. No change to emulator runtime
behaviour — the bios_boot golden hash and the save-state format (v9) are
unchanged, so existing save states still load.

Added

• sdbg reference master-PC trace-diff (tdiff). tdiff <ref> [frames]
  runs ours through the real full-system path and compares the loop-collapsed
  master PC stream against a Mednafen SS_PCTRACE dump, stopping at the first
  divergent PC with a both-sides context window. On a divergence it then
  rewinds to a pre-trace snapshot and re-runs to a breakpoint there, printing
  full registers + the stack call-chain and parking the machine at the
  divergence. Knobs: TDIFF_ADD (Mednafen fetch-PC offset), PCTRACE_LO/HI.
  This hosts the project's primary debugging methodology (the LLE↔Mednafen
  trace-diff) inside the REPL instead of hand-diffing two trace files.
• sdbg multiple breakpoints + symbols. Several register-guarded master /
  slave breakpoints at once (b <addr> [ri v] adds, b lists, bd <id|*>
  deletes), honoured by both c and fc. A symbol table (sym, syms <file>,
  --syms=<file>) resolves names anywhere an address is expected and annotates
  output (name+0xNN) in disassembly, breakpoint hits, and the call-chain.
• Core support: Sh2Entity now holds a set of breakpoints and BpHit
  carries the firing PC (Saturn::set_master_bps/set_slave_bps). Debug-only
  and #[serde(skip)], so the golden and save-state format are unaffected.

Changed

• Documentation layer. Added a Developer tools catalog and a
  References section (authoritative SEGA/Hitachi/Motorola manuals + the
  behavioural oracles, with verified download locations) to CLAUDE.md; merged
  bootstrapping.md into system-architecture.md §9; recorded four ADRs
  (0015 CD-block HLE, 0016 master-leads-slave stepping, 0017 reference-oracle
  policy, 0018 save-state design); and recorded the M12 #9 cycle-accuracy
  residual's cross-emulator corroboration (the post-spin-up seek is Mednafen-only;
  MAME and Yabause match ours — left as-is per the stop rule).