build: make DuckDB 1.5.5 the fleet default

[fuziontech]

Summary

Promotes DuckDB 1.5.5 from canary to the fleet default, following #1028 (which published the 1.5.5 canary images and moved CI/e2e onto 1.5.5).

• Worker CD matrix: 1.5.5 is now default: true in both the build and manifest matrices — it publishes the unsuffixed fleet tags (:latest, :<sha>) and dispatches Charts. 1.5.3 stays in the matrix as a non-default rollback row, still publishing …-duckdb1.5.3 suffixed tags, so rolling a tenant back is a config-store image re-point, and a full fleet rollback is a revert of this PR.
• e2e comment updated to reflect that 1.5.5 now mirrors the default:true row.
• ducklake_version_matrix.sh comment updated (default + rollback versions).

No code changes: go.mod, the Dockerfiles, the e2e/scenario image pins, and the mw-dev harness assertions all moved to 1.5.5 in #1028.

This PR's e2e-mw-dev run is the gate: it builds a 1.5.5 image and exercises the full activation pipeline (control plane → STS → worker pod → DuckDB → DuckLake on real S3/cnpg) on the mw-dev cluster, including the engine-version (v1.5.5) and fork-extension SHA assertions added in #1028.

Known 1.5.5 behavioral note (from #1028 testing): internal catalog schemas (e.g. system, temp) are now listed by duckdb_schemas()/pg_catalog.pg_namespace. No production impact surfaced in the test suite; worth a look during review/canary.

Validation

• Matrix invariants re-verified locally: exactly one default: true (1.5.5) in both matrices.
• build: add DuckDB 1.5.5 canary #1028's merge train on main was fully green (CI, CodeQL, Release Build, all three container CD workflows), and the 1.5.5 worker images published for amd64+arm64.
• Full tests/integration suite and DuckLake concurrency benchmark pass on 1.5.5.

[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	0	0
E2E/journey files	0	0	0
Workflow files	0	2	0

Signals

• Test cases: +0 / -0
• Assertions: +0 / -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.