Check the feed hash before parsing - #1672

Part of #1657 (steps 2b and 2c). Stacked on #1669.

What it does

Two commits:

1. Free the parsed feed before precomputing. storeConfiguredFeed loads, parses and stores the feed, so the parsed copy is reachable only inside it. ReloadStatic calls debug.FreeOSMemory and then precomputes stop directions. This runs only when the data changed, at most once per daily reload.
2. Check the feed hash before parsing. fetchFeedFile downloads a remote feed next to the database while hashing it (a local feed is hashed in place); downloads left behind by a crash are removed first. storeConfiguredFeed asks the new Client.HasImported and parses only when the database does not already hold the feed. StoreGtfsData keeps its own check; both use one isSameFeed rule. The download clones http.DefaultTransport, so proxy environment variables apply.

When the feed is unchanged, both paths call Client.RefreshFeedExpiry, so a database written by an older build still picks up the corrected expiry from #1686 without a re-import.

Root cause

Every startup and daily reload read the whole feed into memory and parsed it before StoreGtfsData compared hashes, so a restart with an unchanged feed reached nearly the same memory peak as a first import. Separately, ReloadStatic kept the parsed feed alive in a local variable while directions were precomputed.

Testing

• Test failing on main before this change: n/a (memory change). The expiry-refresh interaction is pinned by TestReloadStaticUnchangedFeedRefreshesExpiry, which fails without the RefreshFeedExpiry call in the short-circuit:

# maglev.onebusaway.org/internal/gtfs.test
--- FAIL: TestReloadStaticUnchangedFeedRefreshesExpiry (0.13s)
    static_test.go:117: 
        	Error Trace:	/Users/aaron/repos/onebusaway/red-1672/internal/gtfs/static_test.go:117
        	Error:      	Not equal: 
        	            	expected: time.Date(2026, time.January, 1, 7, 59, 59, 0, time.UTC)
        	            	actual  : time.Date(2025, time.December, 31, 7, 59, 59, 0, time.UTC)
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1,2 +1,2 @@
        	            	-(time.Time) 2026-01-01 07:59:59 +0000 UTC
        	            	+(time.Time) 2025-12-31 07:59:59 +0000 UTC
        	            	 
        	Test:       	TestReloadStaticUnchangedFeedRefreshesExpiry
FAIL
FAIL	maglev.onebusaway.org/internal/gtfs	0.344s
FAIL

• Manual check: NJ Transit bus import, TestImportMemory, at the previous PR in the stack and at this branch:

Full per-stage output is in the TestImportMemory log; the table shows the stages this PR moves.

Tests:

• TestFetchFeedFileLocal: a local feed is hashed in place with the expected hash.
• TestFetchFeedFileDownload: the auth header is sent, the file lands next to the database and cleanup removes it; a 403 leaves no file behind.
• TestFetchFeedFileTruncatedDownloadLeavesNoFile: a server that promises more bytes than it sends; the fetch fails and the database directory is empty.
• TestFetchFeedFileServerUnreachable: a closed server gives the download error.
• TestFetchFeedFileRemovesStaleDownloads: a stale download next to the database is removed before a new one starts.
• TestDownloadDir: "", :memory: and a file path.
• TestFeedHashIncludesGTFSTidy, TestParseHashMarksTidyOnlyWhenItRan, TestParseHashMarksTidyWhenItRan: the +gtfstidy marker is added only when tidying ran (the success case uses a fake gtfstidy on PATH).
• TestHasImported: empty database, same feed, other source, other hash.
• TestReloadStaticSkipsParseForImportedFeed: a reload of the same feed does not parse again.
• TestReloadStaticPrecomputesOnlyWhenChanged: directions are precomputed after a changed import and not after an unchanged reload.
• TestReloadStaticUnchangedFeedRefreshesExpiry, TestReloadStaticUnchangedFeedExpiryFailure: the skipped parse still refreshes feed_expires_at; a refresh failure fails the reload without parsing.
• TestReloadStaticHasImportedError: a failing metadata query fails the reload with its error.

Checklist

• This PR addresses one issue.
• It is under ~200 changed lines, or the description says why it can't be split.
• This PR is not a bug fix, or the new or changed test fails on main and passes on this branch.
• Each commit is one logical change with a clear message.
• I did not hand-edit sqlc-generated files under gtfsdb/ (db.go, models.go, query.sql.go) or testdata/openapi.yml (synced from upstream).
• This PR changes no handler, or handlers pass r.Context() to DB/service calls, not context.Background().
• The response comparison above is filled in, or this PR doesn't change a Java-served response.
• This PR adds no endpoint, or each new endpoint matches testdata/openapi.yml, has an issue open on OneBusAway/sdk-config, or is outside the SDK and the description says why.
• I read every line of this diff and can explain it if asked.

Notes for reviewers

Size. 660 added, 134 removed; 367 of the additions are tests. The two commits are separable in principle, but the second moves the code the first introduced, so splitting them would mean reviewing the same lines twice.

Compatibility of the hash.

• Without gtfstidy the hash is still the SHA-256 of the feed bytes, so existing databases match and do not re-import.
• With gtfstidy the hash is now taken before tidying and marked +gtfstidy, only when tidying succeeded. Turning tidy on or off still forces a re-import. A database imported with tidy on re-imports once after this change. A failed tidy is retried on the next reload instead of being recorded as done; while tidy keeps failing, every reload parses the feed again, as before this PR.

Expiry refresh failure. A refresh that fails (for example an agency row with an invalid timezone) fails the reload, in both paths. That matches what #1686 does inside StoreGtfsData.

Follow-up. With gtfstidy on, tidyGTFSData still copies the feed into the default temp directory; it should work in the download directory instead.