fix(cpg): prove the frontend can read the source before building, and check the graph always

Two permanent guards, and a correction that motivates both.

THE CORRECTION. There is no php2cpg defect. Every PHP build on this machine
ran through a local `php` shim proxying stdio through `docker run`, and
php2cpg depends on the INTERLEAVING of its parser's stderr banners with its
stdout JSON to attribute each document to a file. The proxy scrambles it:

    streaming shim   drops 72-88   767KB / 969KB / 1.2MB   different every run
    buffered shim    drops 0       5,703 bytes             empty, and silent
    NATIVE php       drops 0       2,872,996 bytes         identical x3

The 637-file tree with native php: 0 drops, a 4,329,471-byte CPG, 20.0s,
1.9GB peak -- which is exactly what task 5.10 said before I started
"correcting" it. No intermittent failures, no malformed graph, no dataflow
assertion, no scale limit.

GUARD 1: the interpreter must prove it can read the tree, before anything is
built. Every instrument failure this project has had takes one shape -- the
input was unreadable and the tool reported a plausible ZERO -- and it has cost
four wrong diagnoses in a day:

    a php that could not see ~/.cache   -> "cannot handle 637 files"
    a php whose stdio was proxied       -> "drops half its batches"
    a container missing a bind mount    -> "0 dropped", 5,703-byte graph

None announce themselves: file_exists() is false, the parser writes nothing,
the frontend exits 0, and "no errors" reads exactly like success. Checking the
output cannot separate that from an empty repository. Checking that the input
arrived can, and costs one exec.

GUARD 2: the graph census is taken on EVERY build, not only one the frontend
warned about. A graph holding nothing arrives with no warnings at all --
silence is the symptom that has no symptom -- and the shortfall is now
reported as "the frontend reported no failures but the graph holds N of M
source files".

`cpg_build_failed` carries that detail through to the report instead of being
a bare reason.

Also adds tests/test_cpg_queries_compile.py: the queries are 900 lines of
Scala that nothing compiled. A rowsFor signature change shipped with 850 tests
passing and broke every query at runtime, found by a measurement failing hours
later. Each query is now compiled AND run against a real graph wherever joern
exists, and must return a parseable payload.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01T7XD7x5KonMdmGRkp63EQG

src/openultrasast/reports.py

@@ -427,7 +427,7 @@ def _append_coverage(lines: list[str], coverage: Sequence[Mapping[str, object]],
 # silence in the table above is narrower than it looks.
 _COVERAGE_DEGRADATIONS: dict[str, str] = {
     "cpg_empty": "The code graph came back with no methods in it, so **nothing above was analysed at all**.",
-    "cpg_build_failed": "The code graph could not be built, so nothing above was analysed.",
+    "cpg_build_failed": "The code graph could not be built, so nothing above was analysed. {detail}",
     "cpg_sharded": (
         "The graph had to be built in {shards} parts, because the frontend could not parse the whole tree at "
         "once. Each part was queried, but **a flow whose source is in one part and whose sink is in another "