build: add DuckDB 1.5.5 canary

[fuziontech]

Summary

Adds DuckDB 1.5.5 as a non-default canary alongside the shipping 1.5.3, so we can canary tenants onto the new engine without moving the fleet.

• Worker CD matrix: new 1.5.5 row (default: false) pinned to duckdb-go v2.10505.0 / bindings v0.10505.0, with PostHog fork extensions httpfs v1.5.5-cred-refresh-write-retry and ducklake v1.0-posthog.7 (the first DuckLake build for 1.5.5). 1.5.3 stays default: true, so unsuffixed fleet tags and the Charts dispatch are untouched.
• e2e-mw-dev / scenario-dev images now build with the 1.5.5 + new fork tags, so PR e2e runs exercise the canary engine.
• go.mod/go.sum bumped to duckdb-go/v2 v2.10505.0 so local builds and the whole test suite run against 1.5.5.
• mw-dev harness now asserts the DuckDB engine version (v1.5.5) in addition to the bundled-fork SHAs (ducklake 6768849c, httpfs 575da0b).
• ducklake_version_matrix.sh benchmarks 1.5.5 and 1.5.3 by default (and now uses docker compose, since the standalone docker-compose binary is gone from current Docker).

Rollback: revert this PR; 1.5.3 remains the default matrix row throughout.

Validation

• just lint, just test-unit, and the full tests/integration suite pass on 1.5.5.
• DuckLake concurrency benchmark on 1.5.5: 21 metrics, 0 failures.
• Local amd64 builds of Dockerfile and Dockerfile.worker succeed; standalone REPL reports pragma_version() = v1.5.5 with the fork extensions loaded.
• One behavioral note from testing: DuckDB 1.5.5 additionally lists internal catalog schemas (e.g. system, temp) from duckdb_schemas()/pg_catalog.pg_namespace. No production impact surfaced in the suite; flagging in case client catalog queries observe them during canary.

[github-actions]

Test Impact Plan

Deterministic summary of how this PR changes tests, CI runners, and coverage-risk signals.

Summary

Area	Added	Changed	Deleted
Test files	0	3	0
E2E/journey files	0	0	0
Workflow files	0	3	0

Signals

• Test cases: +0 / -0
• Assertions: +1 / -0
• Skips or known failures added: 0
• Workflow continue-on-error added: 0
• Workflow path filters added: 0
• Test commands removed from justfile: 0
• E2E/journey retry lines added: 0

Coverage risk: neutral or increased

No coverage-reduction warnings detected.