feat: deep source provenance, inline query rules, and public env API

[theaspirational]

Summary

Three related additions to the Datalog engine, each independently useful and backward compatible.

---

1. Deep source provenance (dl_get_provenance_src_offsets / dl_get_provenance_src_data)

Replaces the two stub implementations with a full CSR-format source tracking pass that runs after the existing rule-attribution provenance (prov_col).

When DL_FLAG_PROVENANCE is set, for each derived row the engine records which rows in which body relations contributed to the derivation:

Field on dl_rel_t	Type	Meaning
prov_src_offsets	I64[nrows+1]	CSR offsets: offsets[i] = start index in prov_src_data for derived row i
prov_src_data	I64[total]	packed source refs: (relation_index << 32) | row_index

CSR format keeps the result in two flat allocations per relation, lets callers slice out sources for any row in O(1), and mirrors the encoding already used by the engine's CSR edge indices.

Caveat: for rules where a variable appears only in body atoms, that variable is unconstrained during source lookup. Entries may be a superset of the true derivation (all body rows whose head-visible columns match). This is documented in the header. False negatives would be worse.

Both vectors are released in dl_program_free.

---

2. Inline query rules via (rules ...) clause

Adds an optional fourth argument to (query ...) that supplies rules inline, Datomic-style, instead of using the global rule set:

;; Existing behaviour unchanged — uses global g_dl_rules[]
(query db (find ?x ?y) (where (path ?x ?y)))

;; New: inline rules, globals ignored for this query
(query db
  (find ?x ?y)
  (where (path ?x ?y))
  (rules
    ((path ?x ?y) (edge ?x ?y))
    ((path ?x ?z) (edge ?x ?y) (path ?y ?z))))

Each entry in (rules ...) follows the same head+body syntax as (rule ...). When the clause is present only those rules are loaded into the temporary dl_program_t; when absent the existing global-copy path runs unchanged.

This enables multi-database sessions where different queries carry different rule sets without global state pollution, and makes rule sets composable as plain data (lists of lists).

Internally, dl_parse_rule_from_head_and_body is extracted as a shared helper used by both ray_rule_fn and the new inline parse path, removing the duplication.

---

3. Promote ray_env_get / ray_env_set to public API

Adds two declarations to include/rayforce.h:

/* ===== Environment API ===== */
ray_t*    ray_env_get(int64_t sym_id);
ray_err_t ray_env_set(int64_t sym_id, ray_t* val);

Both functions already exist and are widely used internally (src/lang/env.c). Exposing them allows embedders to bind named values into the evaluation environment and retrieve them after eval — the natural way to pass tables into Rayfall queries by name and read results back. Without this, embedders have to link against the internal env.h header directly.

---

Changes

File	What changed
src/ops/datalog.h	Two new fields on dl_rel_t; stub comments replaced with full docs for both getters
src/ops/datalog.c	dl_build_source_prov() (new); dl_parse_rule_from_head_and_body() (extracted helper); inline rules parsing in ray_query_fn; real getter implementations; dl_program_free cleanup
include/rayforce.h	ray_env_get / ray_env_set declarations
test/test_datalog.c	New file: two tests for CSR structure and flag-guard behaviour
test/test_main.c	Registers the new /datalog suite

All 575 tests pass.