gen-orchestrator: emit the per-node bookkeeping once, not per node

[nicolas-maman]

Why

aether-lang-dev/aether-ui#140 adds one .build.ae (tests/min_size) and the whole fan-out fails on both CI legs with:

error: source file exceeds maximum token limit (50000 tokens)
aeb-link: FATAL — failed to compile the fan-out orchestrator

No file is named because the file is aeb's own generated _orchestrator.ae. gen-orchestrator emitted ~360 tokens of bookkeeping inline per node (selector gate, rc capture, status, the telemetry record). Measured with the aetherc lexer over aether-ui's node list: main = 49,412 tokens, #140 = 49,770, against aetherc's 50,000-token cap on a main file (modules get 100,000). The tree was one node from the wall.

What

The bookkeeping is emitted once as two helpers, _aeb_selected(sel, label) and _aeb_record(s, root, records, label, type_word, nrc, start); each node becomes:

if _aeb_selected(_sel, "apps/aevg_anim") == 1 {
    _start = clock_ns()
    _nrc = apps_aevg_anim__D_build_D_ae(s)
    _aeb_record(s, _root, _records, "apps/aevg_anim", "build", _nrc, _start)
}

The extern call stays inline because a fn passed as a value from another TU is DCE'd (aether #2037). Same 138 nodes: 49,770 → 7,227 tokens. Semantics unchanged: same selector gate, same rc→_mark_failed, same record fields, same TU so bldr's cloned statics resolve.

Verified

• Regenerated aether-ui's orchestrator (138 nodes) with the new tool; aetherc --check --lib lib accepts it with the same two bldr-internal warnings as the old one.
• tests/test_gen_orchestrator_budget.ae: generates a 500-node orchestrator (~180k tokens under the old scheme) and requires ae check to accept it against lib/bldr, plus counts that the helpers appear once and every node goes through them. Passes with the new generator, fails 4/4 against the old one.

Follow-up for aether-ui: bump AEB_REF once this is released.