feat(debug): opcode decoding, call-stack walking and range reads

[NullSablex]

Moves into the SDK the AMX facts a debugger had to reimplement on top of it. Every piece here was extracted from the PawnPro Debugger, where it already ran against live SA-MP and open.mp servers (compilers 3.10.10 and 3.10.11).

AmxDbg::function_address (#55, now merged) had no CHANGELOG entry, and neither did Amx::hlw (#54) — both are recorded in this release section, along with the CI and dependency work since the v3.4.0 tag.

Added

• samp::debug::opcode — opcode numbering (the order of the opcode enum in amx.c, identical on SA-MP and open.mp), OP_NUM_OPCODES, the VM's STK_MARGIN, and operand_cells(op) for instruction sizes. Returns None on a variable-length instruction (casetbl) or an out-of-range opcode — the signal for a scanner to stop rather than guess.
• OpcodeMap and Amx::opcode_map() — inverts the VM dispatch table (label address → opcode) to undo the computed-goto rewrite the loader applies on GCC/Clang builds. is_identity() reports a non-relocated image. This replaces the hand-rolled HashMap that docs/vm-debugging.md used to show consumers writing themselves.
• samp::debug::stack::walk and Amx::call_stack(top_cip) — walks the frame chain ([frm] = caller's FRM, [frm + CELL] = return address) into (cip, frm) per frame, top first. The cell reader is injected, so it is unit-testable against a fake memory map and usable host-side; MAX_DEPTH keeps a corrupted stack from spinning a debug hook.
• Amx::read_cells / Amx::read_bytes — range reads with the same amx_GetAddr bounds checking as read_cell. They stop early at the first inaccessible address and return what they read (the natural case at the end of the data segment), returning None only when the start itself is inaccessible; read_bytes needs no alignment. The get_ref-based Buffer/AmxString path needs a function table and cannot serve a debug hook, which is why only single cells were reachable there before.
• Amx::data_only(ptr) — wraps a raw *mut AMX for data-side access only, stating the intent instead of passing a bare 0 as the function table (the usual situation while a VM is paused).

Changelog completeness

The v3.5.0 section covers all 20 commits since the v3.4.0 tag, not just this branch:

• Amx::hlw() (feat(amx): add Amx::hlw() accessor for the heap low-water mark #54) — public API that had no entry anywhere.
• CI: CodeQL advanced setup (ci: migrate CodeQL from default to advanced setup #52), the stable ci-status gate (ci: add stable ci-status gate as the required status check #51), grouped Dependabot (chore: group Dependabot updates into a single PR per ecosystem #44).
• Dependency bumps across cargo, github-actions and pip (deps: bump time from 0.3.54 to 0.3.55 #37, deps: bump sentry from 0.49.0 to 0.49.1 #38, ci: bump github/codeql-action/upload-sarif from 4.37.4 to 4.37.6 #39, ci: bump Swatinem/rust-cache from e18b497796c12c097a38f9edb9d0641fb99eee32 to 258712b0b7b1ddf8bddc9fc3b0faca682b2736c3 #41, ci: bump github/codeql-action/upload-sarif from 4.37.6 to 4.37.7 #42, ci: bump Swatinem/rust-cache from 258712b0b7b1ddf8bddc9fc3b0faca682b2736c3 to f0d9c3887740aee45f6153b24b3a6b815192ec16 #43, chore(deps): bump mkdocs-material from 9.7.6 to 9.7.7 in /docs in the pip group #45, chore(deps): bump github/codeql-action/upload-sarif from 4.37.7 to 4.37.8 in the github-actions group #46, chore(deps): bump pymdown-extensions from 11.0.1 to 11.0.2 in /docs in the pip group #47, chore(deps): bump the cargo group with 3 updates #48, chore(deps): bump memcache from 0.20.0 to 0.21.0 #49, chore(deps): bump the github-actions group across 1 directory with 3 updates #53), none touching the shipped public API.

Validation

• 284 tests + 14 doctests on i686-unknown-linux-gnu; cargo clippy --all-targets -D warnings clean; cargo fmt clean.
• Checked that the new code adds no clippy::pedantic warnings — the project's existing pedantic baseline is untouched.

Docs

• VM Debugging rewritten around the new API, with new sections on opcode numbers/instruction sizes and on walking the call stack.
• API reference updated; README's debug feature description widened and its badge row extended (crates.io version and downloads, docs.rs, MSRV, stars).

Crate versions

Per the project's independent-versioning policy:

• rust-samp (lib samp): 3.3.0 → 3.4.0 — re-exports the widened surface; requires rust-samp-sdk 3.4.0.
• rust-samp-sdk (lib samp_sdk): 3.3.0 → 3.4.0 — additive public API.
• rust-samp-codegen: 1.4.0, unchanged.

Downstream

The debugger already consumes this branch and dropped ~250 lines: its local stack.rs, the 44 OP_* constants, the OP_PARAMS table, its own OpcodeMap, and the manual alignment in its memory reads.