* manifest: create forumpost on demand, matching canonical rebuild

Crosslink's candidate query names the forumpost table unconditionally, but canonical fossil creates it lazily -- only once a forum artifact needs it -- and fossil rebuild drops it for a repository that never populated it. Crosslink now recreates the table itself before the sweep runs, matching canonical's on-demand behavior instead of erroring with "no such table: forumpost" against a rebuilt repo.

Audited the rest of the on-demand-in-canonical set (ticket, ticketchng) against the current schema: both are created eagerly by canonical at init and survive rebuild, so they don't share this problem.

Added TestCrosslinkForumPostTableDropped (drops the table directly) and TestCrosslinkAfterFossilRebuild (drives an actual fossil rebuild first) to cover the regression, and updated the existing fossilbin comparison test's comment now that the rebuild-last ordering is no longer required.

* manifest: visit crosslink candidates in delta-chain order

Crosslink swept candidates in ascending rid order, but content_deltify stores a blob's older versions as deltas against its newer ones, so delta chains run the opposite direction: high-rid root down to low-rid ancestors. An ascending sweep's first candidate of a chain therefore materializes the whole chain, and the rest are consumed arbitrarily far in the future -- the working set of a whole-repository sweep was the entire repository expanded (8.2 GiB for the Fossil SCM repository).

Candidates are now topologically sorted so a delta's base is always visited before its dependents (deltaChainOrder), matching Fossil's own rebuild_step shape: root first, each descendant one delta application from an already-expanded predecessor. The working set this sweep needs is now bounded by how many chains are in flight at once, not by repository size.

That reordering also changes the order artifact side effects get written in, since content expansion and artifact crosslinking share one loop. Most of what Crosslink writes turned out to already be a pure function of its own artifact (event/plink/mlink/tagxref-origin rows) and needed no change. Two things generalize across the whole plink graph rather than any one artifact and were genuinely order-sensitive:

- leaf table bookkeeping, which inserted the new checkin and deleted its parent -- correct only when parents are linked before their children, which delta-chain order no longer guarantees. - tag propagation, which ran once per checkin from its immediate parent -- reachable only to children plink already held at that moment.

Both are now full, order-independent repair passes (repairLeafTable, repairTagPropagation) run once after the sweep, mirroring Fossil's own manifest_crosslink_end + leaf_rebuild() shape: defer the order-sensitive work and repair it once rather than trying to preserve order through the sweep. tag.propagate's mtime-gated, plink-live-at-call-time design turned out to already be confluent -- replaying every self-declared tag origin once, in any order, converges on the same tagxref state -- so the repair is just doing that instead of depending on visiting order to get there implicitly. Tech-note event replacement and forum thread titles turned out to already have the same confluent shape and needed no change; left comments explaining why.

Also fixes a tag.PropagateAll crash on any origin with a NULL tagxref value (e.g. CrosslinkCluster's cluster tag), surfaced by the new propagation repair actually reaching those rows for the first time.

Regression coverage: a real three-generation checkin chain, forced into a genuine manifest delta chain, that would have left two of three checkins untagged and all three marked leaf under the old per-checkin propagation and incremental leaf bookkeeping.

* fix: address review nits on crosslink order-sensitivity and delta cycles

- deltaChainOrder now returns an error instead of panicking when the candidate delta graph fails to drain. The delta table is on-disk data that can arrive over sync from an untrusted peer, so a cycle (corrupt or hostile repo) should surface as an error rather than crash the process.

- snapshotDerived's digest now covers tagxref (rid/tagid/tagtype/ srcid/origid/value), and TestFossilBinaryReadsCrosslinkedRepo compares it against canonical fossil rebuild -- closing the gap the test previously called out by name ("tagxref is the notable gap").

- Added TestFossilBinaryReadsCrosslinkedRepoBranchAndMerge, exercising the order-sensitive paths incrementalHistory's single-branch history cannot reach: a feature branch diverging from trunk, and a merge commit whose own branch tag must come from its primary parent, not the merged-in one. event/plink/tagxref match canonical fossil row-for-row on this topology.

leaf and mlink are deliberately excluded from that comparison: the fixture surfaced that both have pre-existing gaps against canonical fossil (repairLeafTable counts any plink edge rather than only primary-parent edges; insertCheckinMlinks writes a row for every F-card rather than only changed ones) that predate this fix and are out of scope for #102/#103.