Replace Rust toolchain trampolines with soldr

[zackees]

Goal

Replace fbuild's hand-rolled Rust toolchain trampolines (`_cargo`, `_rustc`, `_rustfmt`, `ci/trampoline.py`) with calls to `soldr` v0.7.0.

soldr was designed to solve exactly this problem — it resolves toolchain binaries via `rustup which` so the correct rustup-managed binary is always used without PATH-munging. This removes ~140 lines of fbuild-specific trampoline code.

Scope

In scope — 1:1 behavior-preserving migration:

• `_cargo` → `exec soldr --no-cache cargo "$@"`
• `_rustc` → `exec soldr rustc "$@"`
• `_rustfmt` → `exec soldr rustfmt "$@"`
• `ci/trampoline.py::cargo/rustc/rustfmt/clippy_driver()` — call soldr instead of PATH-munging
• `ci/trampoline.py::_run_cargo_bin()` (`run_fbuild`, `run_fbuild_daemon`) — route through `soldr --no-cache cargo run`
• `ci/hooks/tool_guard.py` — accept `soldr ` as a valid prefix
• Add `soldr>=0.7.0` to `ci/dev-tools` dependencies
• Doc updates in `CLAUDE.md`, `CODEX.md`, `ci/dev-tools/README.md`, `ci/hooks/README.md`

Out of scope for this PR:

• Dropping the `zccache>=1.2.11` Python dep (fbuild may still need it standalone)
• Removing `ci/env.py::activate()` / `clean_env()` (legacy callers exist; leave for a later cleanup pass)
• Adopting soldr's built-in zccache RUSTC_WRAPPER path (`soldr cargo` without `--no-cache`) — that's a deliberate "do we want a compilation cache?" decision, not a side-effect of this trampoline swap. Keeping `--no-cache` preserves existing build semantics.
• Migrating ecosystem tools (`cargo-nextest`, etc.) to `soldr cargo <sub>` — follow-up</sub>

Why now

soldr v0.7.0 shipped (https://github.com/zackees/soldr/releases/tag/v0.7.0) with the full Rust toolchain passthrough surface:

• `soldr rustc`, `soldr rustfmt`, `soldr clippy-driver`, `soldr rustdoc`, `soldr rust-gdb`, `soldr rust-lldb`, `soldr rust-analyzer` — all resolve via `rustup which`
• `soldr cargo ` — cargo front door, honours `rust-toolchain.toml`, forces MSVC on Windows native builds without flagging
• `--no-cache` top-level flag disables soldr's compilation-cache wrapper for parity with a bare cargo invocation

Acceptance criteria

• `uv run cargo check --workspace` still succeeds with the same result as before
• `uv run cargo clippy --workspace` unchanged
• `uv run rustfmt --check` unchanged
• `./_cargo`, `./_rustc`, `./_rustfmt` still work from bash
• `ci/hooks/test_tool_guard.py` passes with new allowed prefixes
• `run_fbuild`, `run_fbuild_daemon` console scripts still launch the CLIs
• All existing GitHub Actions builds green on the PR

Risk / rollback

Minimal. soldr's rustup resolver mirrors what fbuild's trampolines already do. If anything goes sideways on the PR, revert is a single-commit rollback; fbuild's existing trampoline logic stays intact in the reverted commit history.