feat: soft-deprecate tree-walker (remove --run-tree CLI, internalise interpreter module)

[danieljohnmorris]

Summary

The tree-walking interpreter has been the slowest engine since the VM became the default (PR #390). The --run-tree flag was mostly a foot-gun: agents reached for it for debuggability and silently paid an 8x wall-clock tax on real workloads. This soft-deprecates the engine without ripping it out, picking the safe path through the blocker called out by the 2026-05-18 STOPPED entry on ilo_assessment_feedback.md (14 tree-bridge consumers + 191 references to shared infra still living under src/interpreter/).

Result: agents pick from VM / JIT / AOT, the tree-walker keeps running internally for the HOF / regex / fmt / IO shapes the VM hasn't lifted natively yet, and the door is open for a clean hard-drop in 0.13.0+ once the bridge consumers and shared runtime types get extracted.

What's in the diff

Four commits, each a coherent slice:

1. cli: drop --run-tree / --run from the public engine surface — src/cli/args.rs strips the two flags from the clap derive (they fall through to the existing unknown-flag guard), src/lib.rs gates interpreter behind #[doc(hidden)], src/main.rs removes the tree branch from extract_run_engine_flag and the bare-args dispatcher, and the wr 3-arg dynamic-format compile error stops suggesting --run-tree as the workaround.

2. tests: rewrite cli flag-placement / unknown-guard for the --run-tree removal — cli_run_flag_placement.rs got a full rewrite (per-position tests now exercise --run-vm; the conflicting-flags tests pin --run-vm vs --jit; the --run alias tests assert clean rejection via the unknown-flag guard). regression_unknown_flag_guard gains a positive test that --run-tree exits non-zero with the flag named in stderr.

3. tests: sweep --run-tree to --run-vm across the regression suite — 159 test files. ENGINES_ALL arrays shrink from [tree, vm, jit] / [tree, vm] to [vm, jit] / [vm]; single-element loops trip clippy's single_element_loop lint where they used to be multi-engine, so the affected files gain a file-level #[allow] with a pointer comment. coverage_interpreter widens its error-code asserts because tree and VM use different ILO-R### codes for the same shape errors; wr_unknown_format_errors accepts the VM's undefined function compile-time refusal since the VM has no native 3-arg wr-with-dynamic-format path.

4. docs: sync SPEC / ai.txt / SKILL / README for --run-tree removal — drops --run-tree from in-process runner lists, retires the stale Phase-2-closures-only-on-tree sentence (VM/JIT have handled captures natively since PR1/PR2), adds a Tree-walker is internal-only paragraph naming the remaining bridge consumers, and removes the ilo Interpreter row from the README benchmarks table.

Companion commit on the site repo: ilo-lang/ilo-site@c2d6fec drops --run-tree from engines.md, cli.md, and benchmarks.md. Already pushed; the site has no CI so no follow-up there.

Before / after

$ ilo --run-tree 'f>n;5' f
error: unrecognised flag '--run-tree'. Use 'ilo --help' for valid flags. To pass it as a literal arg, separate with '--' first.
exit=1

$ ilo --run 'f>n;5' f
error: unrecognised flag '--run'. Use 'ilo --help' for valid flags. To pass it as a literal arg, separate with '--' first.
exit=1

$ ilo --run-vm 'f>n;5' f
5
exit=0

Why soft-deprecate rather than hard-drop

The 2026-05-18 STOPPED entry catalogues why the hard-drop was blocked: is_tree_bridge_eligible in src/vm/mod.rs:532 still routes 14 builtin-shape pairs through call_builtin_for_bridge + ACTIVE_AST_PROGRAM TLS (Rgx 2, Rgxall 2, Rgxall1 2, Fmt, Rd 2, Rdb 2, Sleep 1, Ct 2/3, Rsrt 2/3, Map 3 ctx, Flt 3 ctx, Fld 4 ctx, Srt 3 ctx). The interpreter:: module also exports shared infra used by VM and Cranelift (Value, MapKey, RuntimeError, math helpers) — 191 references across 8 files.

Soft-deprecation keeps both of those working while removing the user-facing flag and the rustdoc / IDE-completion noise. 0.13.0+ can hard-drop after PR3d/PR3e migrate the bridge consumers natively and the shared runtime types get extracted to a non-engine module.

Test plan

• cargo build --release --features cranelift clean
• cargo test --release --features cranelift — 6933 passed / 0 failed
• cargo fmt --all -- --check clean
• cargo clippy --release --features cranelift --all-targets -- -D warnings clean
• Manual smoke: --run-tree, --run rejected with clean errors; --run-vm still works
• CI green on push

Follow-ups

• 0.13.0+: native lift of the remaining tree-bridge consumers (3-arg ctx HOFs, regex/fmt/io), extract Value / MapKey / RuntimeError and math helpers to src/runtime/, then hard-drop src/interpreter/.
• ilo-assessment-feedback ## ✅ Addressed move on merge.

[codecov]

❌ 3 Tests Failed:

Tests completed	Failed	Passed	Skipped
4517	3	4514	0

View the top 3 failed test(s) by shortest run time

ilo::regression_aot_strconst_interning::aot_two_functions_each_with_string_constant

Stack Traces | 5.12s run time

thread 'aot_two_functions_each_with_string_constant' (36213) panicked at tests/regression_aot_strconst_interning.rs:89:9:
assertion `left == right` failed: two-fn-two-strconst/--run-tree: stdout diverges. got="" expected="world\n"
  left: []
 right: [119, 111, 114, 108, 100, 10]
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

ilo::regression_aot_strconst_interning::aot_sum_type_pick_compiles_and_runs

Stack Traces | 5.65s run time

thread 'aot_sum_type_pick_compiles_and_runs' (36183) panicked at tests/regression_aot_strconst_interning.rs:89:9:
assertion `left == right` failed: sum-type-pick/--run-tree: stdout diverges. got="" expected="cat\n"
  left: []
 right: [99, 97, 116, 10]
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

ilo::regression_aot_strconst_interning::aot_many_functions_many_strconsts

Stack Traces | 6.95s run time

thread 'aot_many_functions_many_strconsts' (36159) panicked at tests/regression_aot_strconst_interning.rs:89:9:
assertion `left == right` failed: many-fn-many-strconst/--run-tree: stdout diverges. got="" expected="a-1b-2!\n"
  left: []
 right: [97, 45, 49, 98, 45, 50, 33, 10]
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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[danieljohnmorris]

Abandoning this PR — main's skill docs (SKILL.md, ilo-engines.md, ai.txt, SPEC.md) have restructured substantially since this branch was opened, leaving the doc-sync commit obsolete and producing 135-line semantic conflicts. Redoing fresh off current main is cleaner than merging through the drift. New PR coming under feature/soft-deprecate-tree-walker-v2.