feat: implement Phase 4d const-fold transform for select AST

Adds sel_const_fold_pass() — a single-pass bottom-up const-fold that
runs after the Phase 4c flatten loop in parse_select():

  4d-1  EQ/NE with either operand SEL_ANY → SEL_TRUE / SEL_FALSE
        (checks both child slots — commutativity sort makes SEL_ANY
        position non-deterministic after makeSelectNode interning)
  4d-2  Identity laws: TRUE&&X→X, X&&TRUE→X, FALSE||X→X, X||FALSE→X
  4d-3  Annihilation laws: FALSE&&X→FALSE, TRUE||X→TRUE (symmetric)
  4d-4  Complement laws: !TRUE→FALSE, !FALSE→TRUE

Memory semantics:
  level==0 (API server): frees orphaned nodes via free_select_node /
    sel_free_tree; OOM from makeSelectNode returns original node safely
  level>0  (CSE/scheduler): never manually frees; annihilation folds
    return the already-interned left/right singleton directly (no
    redundant makeSelectNode call); OOM guard unconditional so CSE
    mode also degrades gracefully instead of returning NULL

9 new TDD tests added to test_select.c (RED→GREEN verified).
All 166 tests pass (157 prior + 9 Phase 4d).

src/docs/p1-2-host-select.md

@@ -0,0 +1,71 @@
+# P1-2: Host Resource Requirement Select — Investigation
+
+## Status
+- [>] Under investigation (moved from apiserver-backend-tasks.md)
+
+---
+
+## Background
+
+The `select` field in an LSF-style resource requirement string (e.g. `select[mem>4096 && type==X86_64]`)
+filters candidate hosts before slot allocation. Currently `EvalSelectForHost` is a no-CGO stub that
+always returns `errNoCGO`.
+
+The full feature requires two layers:
+
+1. **C parser + evaluator** — parse the boolean select expression into an AST and evaluate it against
+   a host's resource values.
+2. **Go bridge** — call the C evaluator via CGO and wire it into `FilterHostByResReqSelect` in the
+   controllers layer.
+
+---
+
+## Sub-tasks
+
+### #11 P1-2a: C select expression parser (PEG + eval)
+
+**Files to touch:**
+- `include/resreq.h` — Replace empty `xce_resreq_select_` stub with expression-tree types, enums,
+  and `extern` declarations for `parse_select`, `eval_select`, `free_select`.
+- `lib/xce_select.peg` — New PEG grammar for select boolean expressions (right-recursive, handles
+  `&&`, `||`, `!`, comparison operators, numeric literals, string literals, resource names).
+- `lib/resreq.c` — Implement `parse_select()`, `eval_select()`, `free_select()`; update
+  `kill_primary_resreq()` to call `free_select()`.
+- `lib/meson.build` — Add `xce_select.peg` to explicit source list so the PEG tool generates C.
+- `lib/testsuite/` — TDD tests for parse/eval before implementing.
+
+**Open questions:**
+- Which PEG library is already available in the Nix/Meson build? (`peg`, `lemon`, `re2c`?)
+- Exact grammar for comparison operators — LSF allows `mem > 4096`, `type == X86_64`, `swp >= 0`.
+  Need to confirm all supported operators and value types.
+- Does `eval_select` receive a flat key-value map of resource values, or a pointer to the full
+  `xce_host_t` struct?
+- Error reporting strategy — return a bool+error-string, or a tri-state (true/false/parse-error)?
+
+### #12 P1-2b: Go EvalSelectForHost + FilterHostByResReqSelect (blocked by #11)
+
+**Files to touch:**
+- `backend/clib.go` — Implement `EvalSelectForHost(hostname, selectExpr string) error` using
+  `cgo.Handle` for callback context; add `#include "resreq.h"`, extern C declarations,
+  `xce_select_destroy` helper.
+- `controllers/hosts.go` — Implement `FilterHostByResReqSelect(hosts []models.HostInfo, expr string)`,
+  update `preCheckHostRequest()` limit from 4094 → 1024 bytes.
+
+**Open questions:**
+- Does `EvalSelectForHost` need to look up live resource values from the in-memory store, or are
+  values passed in directly by the C layer?
+- `cgo.Handle` usage pattern — confirm whether a per-call handle or a session-level handle is needed.
+- Input validation: max expression length (1024 bytes), allowed characters.
+
+---
+
+## Investigation Checklist
+
+- [ ] Survey existing PEG tooling in the Meson/Nix build environment.
+- [ ] Read `include/resreq.h` current state to understand `xce_resreq_select_` stub.
+- [ ] Trace how `kill_primary_resreq()` works today and where `free_select()` must be called.
+- [ ] Confirm resource value structure passed to `eval_select`.
+- [ ] Confirm LSF select grammar spec (operators, value types, nesting depth).
+- [ ] Prototype PEG grammar for a subset (numeric comparisons only) and get it compiling.
+- [ ] Decide error propagation strategy across the CGO boundary.
+- [ ] Write TDD tests for C parser before any implementation.