Add --allow CODE to exclude specific problem codes from validate's pass/fail decision

Problem

sil-lift validate --strict is meant as a CI conformance gate (see the interop guide), but real-world FieldWorks/FLEx exports trip several warning-level findings that are expected FLEx quirks rather than defects — uri-not-rfc being the clearest example (documented policy in docs/en/guides/validate.md). Right now the only lever is --no-check-media, which is hardcoded to one specific code (missing-media) and fully suppresses it from output rather than just excluding it from the strict decision.

A caller who wants --strict for genuine regressions but doesn't want it to fail on a known-tolerated code (e.g. uri-not-rfc) currently has no way to express that short of post-processing --format json output themselves.

Proposed solution

Add a repeatable --allow CODE flag to validate:

sil-lift validate export.lift --strict --allow uri-not-rfc

Semantics: problems whose code is in the allow-list are still collected and printed/emitted (so a human or a CI log still sees them), but they're excluded from the errors/warnings counts that decide the exit code and from --strict escalation. This differs from --no-check-media, which drops missing-media findings entirely.

Sketch of the affected logic in _cmd_validate (src/sil_lift/_cli.py):

def _cmd_validate(args: argparse.Namespace) -> int:
    problems = _collect_problems(args)
    allowed = set(args.allow)
    counted = [p for p in problems if p.code not in allowed]
    errors = sum(1 for p in counted if p.level == "error")
    warnings = len(counted) - errors
    failed = bool(errors) or (args.strict and bool(warnings))
    ...

For --format json, add an additive summary.allowed count alongside the existing errors/warnings.

Open questions

• Scope: should --allow apply to error-level codes too (e.g. the trait/field-in-range-element schema errors that are deliberately kept as errors per policy), or warnings only? The mechanism above is symmetric either way — it's a decision about what we want to invite people to silence.
• SemVer: --format json's shape is a documented, tested interface (see docs/en/guides/lift-export-interop.md). Adding summary.allowed is additive/safe for a minor bump but should be called out explicitly in the CHANGELOG since CI consumers may assert on summary's exact keys.
• Unknown codes: should --allow on a code that never appears (typo, or a code that doesn't exist) warn/error, or silently no-op? Leaning toward silent no-op for forward-compatibility (a code retired in a later version shouldn't break an existing --allow list).

Alternatives considered

• A dedicated --flex flag that downgrades a fixed, tool-chosen set of "FLEx-known" warnings to an info level. Rejected: uri-not-rfc is the only warning that's genuinely FLEx-specific under current policy (missing-media, undefined-range-value are generic, source-agnostic data problems a gate might legitimately want to fail on), and introducing a third Problem.level value widens the documented/SemVer-covered JSON schema for one code's benefit. --allow CODE covers the same need generally, without the tool prescribing what counts as "FLEx-known."

Start in src/sil_lift/_cli.py at _cmd_validate and trace validate's argument parsing, problem counting, strict exit decision, and JSON summary output. Read docs/en/guides/validate.md and docs/en/guides/lift-export-interop.md for the documented behavior and interface; done means repeatable --allow CODE preserves emitted findings while excluding matching codes from decision counts, with the additive allowed summary count and documented open-policy decisions addressed.