feat(conflict): Add MySQL backed target signature store

[manjari25]

Why?

Signatures are expensive to recompute, so the analyzer needs a durable source of truth keyed by batch. This PR implements the SignatureStore contract from the previous PR on MySQL, as a package owned by the tango conflict extension.

What?

Everything is added under submitqueue/extension/conflict/tango/mysql:

• Adds the target_signature table:
  • Primary key (queue, batch_id), with no secondary index.
  • A row is written once and never updated.
  • targets is a LONGBLOB, because the largest signatures can be large.
• Adds a queue-bound MySQL SignatureStore:
  • A duplicate create maps to storage.ErrAlreadyExists.
  • A signature whose queue differs from the bound queue is rejected.
  • Get reports a missing batch as found=false, not as an error.
• Adds a small binary codec for the targets: an 8-byte big-endian ID followed by a signed varint distance per entry. The codec lives next to the MySQL store because the byte format is a storage detail, so callers only ever see tango.TargetSignature.
• Adds the schema directory to the queueshard linter so the new table's primary key is checked.
• Restricts the Bazel visibility of the MySQL package to the tango tree, the orchestrator service wiring, and its integration test.
• Adds an integration suite with its own compose file, which runs the store against a real MySQL. It covers a large (100k-target) signature, a duplicate create keeping the first signature, and queue isolation.
• The store is not added to the orchestrator Storage aggregate. Nothing constructs it yet, and wiring the schema into the compose stack, the e2e suites and the local init target is left for the PR that uses it.

Test Plan

./tool/bazel test //submitqueue/extension/conflict/tango/mysql:go_default_test
./tool/bazel test //test/integration/submitqueue/extension/conflict/tango/mysql/... --test_output=streamed
make check-gazelle
make check-mocks
make lint
CI

Issue