* refactor(vfile): give the checkout's vfile encoding one owner (#233)

Checkin, stash and status each decoded fossil's vfile columns on their own: rid=0 for an add, origname==pathname on unrenamed rows, chnged merge codes. That is how #228 happened, and stash still had it wrong: it selected chnged=1 only, so files changed by a merge (chnged 2..9) were silently left out, and it stashed a rename as a plain edit.

New package internal/vfile owns the encoding. Load reads a version's rows; Row answers what callers ask (IsAdded, IsRemoved, RenamedFrom, ContentChanged, PendingMerge); Classify gives the change type in fossil's order. checkout.FileChange is now an alias of vfile.Change, so the checkout API is unchanged.

internal/checkout's status, Extract safety check and checkin read rows through it. The never-read vfileCommitEntry.changed field is gone.

internal/stash selects changes with vfile.Classify against the checked- out version and refuses a pending merge or rename by name, since its save and revert paths restore files by current name against the committed version and would drop either. Its store-and-revert loop is split into stashRecord and revertStashedFile; removed and modified files share one revert path, keyed by row id.

* fix(vfile): a pending merge is chnged 2-5 or mrid != rid (#233)

PendingMerge counted every chnged above 1 as a merge, but 6 through 9 record a change to the executable or symlink bit; fossil leaves chnged=6 after a plain chmod +x. Stash then refused an ordinary permission change as "a pending merge". It also ignored mrid: an edit made on top of a merge drops chnged back to 1 while mrid still names the merged-in content, so the refusal could be bypassed and the merge half-dropped.

Row now carries mrid; PendingMerge is chnged 2..5 or mrid != rid, and ModeChanged covers 6..9. Stash refuses a permission change with that reason, since its revert writes default permissions.

Load reads a NULL rid as 0 and a NULL mrid as rid, the tolerance the checkin code had when it scanned rid into a NullInt64.

* refactor(checkout): ask vfile.Row about merges in Extract's check (#233)

Extract's safety check relaxed only rows with chnged == 1, decoding the column itself. It now asks the row (not a pending merge, not a mode change), keeping fossil's encoding in internal/vfile. Equivalent for every real row. nextChangedState's comment no longer calls the mode changes (6..9) merge states. check-in: f55d0ed2de user: [email protected] tags: trunk