A persistently failing file makes every incremental update a full rebuild (identity-version loop)

[merlincat11]

Split out of #939 review. Reachable on main today for C++; #939 would have extended it to Java/C#.

Problem

full_build refuses to record cpp_identity_version when any C++ file failed to parse,
because a failed file keeps its old-format edges:

if not cpp_errors:
    store.set_metadata(_CPP_IDENTITY_METADATA_KEY, CPP_IDENTITY_VERSION)

incremental_update then sees a stale identity version and triggers full_build. If the
file fails every time — genuinely malformed, unreadable, or hitting a parser bug — the
marker is never recorded and every incremental update becomes a full rebuild,
indefinitely.

Reproduction

Simulated a Java file that always raises during parse, using the #939 branch (same logic,
extended to Java/C#):

inherits_identity_version after failed build: None
  update 1: identity_rebuild = True
  update 2: identity_rebuild = True
  update 3: identity_rebuild = True

Substituting a C++ file reproduces the same loop against main.

Why it matters

The guard is correct in intent — marking the version current while a file retains stale
edges would mean never retrying it. But the current shape converts one bad file into a
permanent, repo-wide performance cliff, and it is silent apart from an INFO log.

Possible directions

• Record the version and track the specific failed files, so only those are retried
  rather than rebuilding everything.
• Record the version after a rebuild attempt regardless, accepting that a permanently
  unparseable file keeps stale edges (it has no valid edges to offer anyway).
• Bound retries, e.g. attempt the identity rebuild once and record the attempt.

Worth fixing for C++ and any future identity version together, so the mechanism has one
agreed behaviour.