feat(testkit): CI guardrails + headless runner for compiler validation

[begeistert]

Description

Makes RP2040Sharp a robust firmware/compiler testkit (e.g. for PyMCU) that never flakes
or hangs in CI, and prepares the repo for the 1.0.0 stable release.

TestKit CI guardrails

• RP2040TestSimulation.RunUntilHalt(predicate, maxInstructions) — a bounded run that
  cannot hang; returns a diagnostic RunResult (PredicateMet / LockedUp /
  BudgetReached) so wedged or crashed firmware fails the test with a reason instead of
  stalling the job.
• CPU-health assertions: NotBeLockedUp(), NotHaveFaulted(), BeInThreadMode(),
  HaveExecutedAtMost(n).
• InstructionCount — deterministic, reproducible metric for compiler-size regressions.

Headless runner CLI (src/RP2040Sharp.Runner)

• Loads a UF2/bin, runs under a hard instruction budget, watches UART/USB-CDC for
  --expect-text, and exits 0 (found) / 1 (not found) / 2 (crashed). The rp2040js-style
  --expect-text workflow, headless. Verified end-to-end against hello_uart.uf2.

Release prep

• Repository URLs pointed at the PyMCU org (RepositoryUrl/PackageProjectUrl, README,
  CHANGELOG links). Still published by begeistert.
• CHANGELOG closed for 1.0.0.

Tests: 453 unit + 149 integration, all green. Adds 8 CiGuardsTests (incl. a UDF
double-fault lockup case). The runner is IsPackable=false, so dotnet pack still emits
only RP2040Sharp and RP2040Sharp.TestKit.

Type of change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 🧹 Refactoring (no functional changes, just code cleanup)

Checklist

• 🧪 Tests passed locally (dotnet test)
• ✅ Added new tests for this feature/fix
• 📝 Documentation updated (if applicable)
• 🚫 No "Magic Strings" or hardcoded values
• 💾 Validated against the RP2040 Datasheet (if applicable)

Screenshots / Hex Dumps (Optional)

Runner exit codes (verified):

hello_uart.uf2 --expect-text "Hello"   → exit 0  (OK: found "Hello" after 6810 instructions)
... --expect-text "NEVER"              → exit 2  (firmware crashed)
missing.uf2                            → exit 66 (no input)
(no args)                              → exit 64 (usage)