build: add GitHub fallback for SQLite source archive

[marcus-pousette]

Why

The primary sqlite.org release archive has intermittently returned 503 errors in CI. A matching GitHub release-tag archive gives the build a deterministic fallback without changing the SQLite source version.

What changed

• Build SQLite 3.53.0 from the official sqlite-src-3530000.zip archive first.
• Fall back to GitHub's version-3.53.0 tag archive.
• Detect either archive's extracted root before running configure.

Stack

This PR is based on #4, which synchronizes the fork with upstream wa-sqlite 1.1.1. #2 then adds the failed-open cleanup on top.

Verification

• Forced the GitHub URL as the only source and generated the SQLite 3.53.0 amalgamation successfully.
• The Sync upstream wa-sqlite 1.1.1 #4 checked-in browser baseline remains green: 2,895 passed, 0 failed. This PR changes only source-archive acquisition in the Makefile.
• The dependent TreeCRDT #210 CI successfully fetches SQLite 3.53, builds the extension-linked WASM, and passes the full integration suite.

Merge method

Use Create a merge commit after #4. The dependent submodule stack refers to this exact commit ancestry.

[marcus-pousette]

Closing this optional download fallback to keep the required stack minimal. PR #2 has been restacked directly onto the upstream 1.1.1 sync in #4, so this PR is no longer part of the merge path.