Phase 5 corrections: sharpen 5.1's boundary claim, fix hosted Makefile's own stale VERSION

Advisor review of the Phase 5 close-out commit caught two real issues:

- Task 5.1's write-up claimed the Isabelle pass "confirms no regression" --
  overstated. Nothing in proof/'s scope changed, so the pass isn't
  regression evidence, it's a build-completeness formality; the boundary
  argument alone already supports the real conclusion. Sharpened.

- sbom.spdx was regenerated before the 5.4 version bump, so it briefly
  understated the engine version. Root cause: the hosted Makefile carries
  its own separate hardcoded VERSION (Makefile:17, still 3.1.0), distinct
  from Makefile.starkernel's copy that 5.4 bumped -- duplication CLAUDE.md's
  own "two independently tracked version strings" note doesn't document.
  Bumped to 3.2.0 to match, regenerated (PackageVersion now correct), and
  rebuilt the hosted starforth binary so lfs/amd64/starforth reflects it.

Also recorded why 5.1-5.4 landed as one commit (deviation from this
document's per-task-commit discipline) and sharpened the riscv64 log
entry so the FAILED run and the accepted retry aren't ambiguous to a
future reader grepping logs/.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Makefile

@@ -14,7 +14,7 @@
 # CONFIGURATION
 # ==============================================================================
 
-VERSION ?= 3.1.0
+VERSION ?= 3.2.0
 CC = gcc
 
 # Isabelle configuration