* fix: distinguish absent, empty, and valued U-cards in manifest parsing

Manifest parsing collapsed a wholly absent U-card and a present-but-empty U-card into the same stored value (empty string), so every crosslink call site that wrote the check-in user into event/attachment/forumpost rows lost the distinction between "no login recorded" (should be SQL NULL) and "login explicitly blanked" (canonical fossil resolves this to the literal "anonymous" at parse time — see src/manifest.c:1008-1016).

Change deck.Deck.U from string to *string so the three states are representable: nil (absent), non-nil "anonymous" (present-empty, resolved in parseUCard the same way fossil's own parser does), and non-nil with a value (present). Resolving once in the parser means every downstream crosslink/rebuild call site can keep binding d.U directly — Go's database/sql converts a nil *string argument to SQL NULL and dereferences a non-nil one automatically, so no per-call-site special casing is needed. The type change itself makes the old ambiguous string-binding path a compile error everywhere Deck.U is constructed, closing off the drift that caused issue #29.

Added deck.Str, a small pointer-literal helper, since Go can't take the address of a string literal directly; existing tests that built Deck{U: "..."} now go through it.

* fix: review nits — whitespace U-cards, rebuild coverage, tag.AddTag, naming

Four follow-ups from review of the U-card presence fix:

- deck.parseUCard now trims args before the emptiness check, matching canonical fossil's next_token() (skips leading whitespace) and every sibling card parser in this file. Previously "U \n" stored literal whitespace instead of resolving to "anonymous". Added a whitespace-only case to TestParseUCardStates.

- Added TestRebuild_UCardPresenceStates covering the rebuild path (rebuild_manifest.go's rebuildCheckin), mirroring TestCrosslink_UCardPresenceStates. The binding site was already correct (same nil-to-NULL pointer semantics as crosslink.go) but had no dedicated regression test.

- tag.AddTag now panics on an empty opts.User, matching the panic idiom CheckinOpts.User already establishes. Previously a tag created with an empty User silently crosslinked to user=''; that asymmetry with CheckinOpts was a wart worth closing rather than documenting as intentional.

- Renamed deck.Str to deck.User: the name should say what the helper constructs (a Deck.U value), not how it's implemented (a pointer take). check-in: 527759acd5 user: [email protected] tags: trunk