Add CPython 3.15 support to mixed-mode debugging

[rchiodo]

Summary

Adds CPython 3.15 support to the mixed-mode (Python/Native) debugger, building on the 3.14 work in microsoft#8586. It makes the _Py_DebugOffsets reader version-aware so it can parse both the 3.14 and 3.15 offset-table layouts, and adds the V315 language version so the 3.14+ code paths (_PyStackRef masking, in-process line-number decoding) fire for 3.15.

Background

CPython 3.15 keeps the self-describing _Py_DebugOffsets table introduced/formalized in 3.14, but the flat layout of the table grew, which breaks a parser that assumes the fixed 3.14 layout:

• thread_state gained 7 fields (3 inserted after current_frame: base_frame, last_profiled_frame, last_profiled_frame_seq; 4 appended after status: holds_gil, gil_requested, current_exception, exc_state) — 9 → 16 fields.
• A new err_stackitem group (exc_value) is inserted between thread_state and interpreter_frame.
• type_object gained tp_basicsize / tp_dictoffset; a new heap_type_object group (size, ht_cached_keys) follows it.
• unicode_object gained compactunicodeobject_size; gc gained frame, generation_stats_size, generation_stats.

Because groups after thread_state shift, the flat offsets of interpreter_frame / code_object in the table differ between 3.14 (table size 760) and 3.15 (table size 888). The parser must select the layout by version.

The interpreter-frame field offsets, _PyStackRef decode (low-bit tag mask), and the co_linetable format are unchanged in production 3.15 builds, so the existing 3.14+ masking and line-number code applies to 3.15 as-is.

What this does

Version-aware _Py_DebugOffsets reader:

• PyDebugOffsets now parses the header/version first, then selects the matching layout (Layout314, Layout315) built from shared field arrays plus version-specific arrays, and reads that layout's table size. Unknown versions are rejected.
• Is314 is joined by Is315, and callers gate on the new IsSupported (3.14 or 3.15) instead of Is314. TableSizeFor(major, minor) replaces the single static table size; TryRead reads the larger MaxTableSize.

Language-version wiring (required):

• PythonLanguageVersion.V315 = 0x030f and GetPythonLanguageVersion handles "315". Without this, LanguageVersion would be None, the >= V314 localsplus / f_executable stackref masks would never fire, and Debug.Assert(version != None) would trip — producing empty locals / broken frames exactly like pre-3.14.

No hot-path map change:

• DebugOffsetsFieldProvider needs no mapping change: every hot-path struct/field it reads (_PyInterpreterFrame, PyCodeObject, _ts) exists in both layouts; only its doc comment was refreshed.

Files

• Proxies/Structs/PyDebugOffsets.cs — version-aware layout (Layout314 760B / Layout315 888B), GetLayout, Is315, IsSupported, TableSizeFor, version-selecting TryParse, TryRead reads MaxTableSize.
• PythonRuntimeInfo.cs — case "315" → V315; StructFieldOffsetProvider gates on IsSupported.
• Common/Parsing/PythonLanguageVersion.cs — V315 = 0x030f.
• Proxies/Structs/DebugOffsetsFieldProvider.cs — doc comment only (map unchanged).
• Tests/DebuggerTests/PyDebugOffsetsTests.cs, PyDebugOffsetsProviderTests.cs — TableSizeFor; synthetic 3.15 structural vector with ordinals derived independently from the CPython 3.15 header; 3.15 provider test; unknown-version rejection.

Testing

• Unit tests pass (18/18 parser + provider tests, including the new 3.15 structural/provider tests and unknown-version rejection).

Notes

• The 3.15 tests currently use a synthetic (structurally correct) offset vector. Adding a real recorded 3.15 _Py_DebugOffsets vector (standard and free-threaded 3.15t), captured in-process the same way as the 3.14 vectors, is a planned follow-up once broader end-to-end 3.15 validation is done.
• As with 3.14, only the mixed-mode hot path is driven by the table; struct sizes and all non-hot-path fields still come from the PDB. Free-threaded (3.15t) layout shifts are covered by the table automatically.

[rchiodo]

Superseded by microsoft#8611 (this one accidentally targeted the fork's own main instead of upstream microsoft/PTVS).