Spine made the stored history append-only while several paths still
assumed stored == projected:
- repair the spine tree when undo/clear truncates the history beneath
it (new spine.truncate_repair op): clamp straddling spans, void
truncated ones, re-anchor the epoch boundary and the observation
cursor, so post-undo messages are never folded away and the next
tree transition is never dropped
- estimate the folded view instead of the raw stored history for the
context-size gauge after undo and on resume, and for 413 overflow
heuristics; fixes spurious auto compaction after every undo
- summarize only the current spine epoch (chaining the previous
epoch summary) instead of the full append-only history
- pause auto compaction with a warning when the compacted shape
still exceeds the window; recalibrate on any model window change
- share the projected-estimate helper via
IAgentContextProjectorService.estimateProjectedTokens
Also records the undo/spine/compaction semantics in FormularDefTree/.
