fix(chart): keep the live edge free of unread spans

[mrcsin]

Problem

A pen in follow mode lost samples the viewer had received or the archive held, and the line drew one
straight segment across the hole until a gesture read the window again. Three defects caused it:

• A history apply replaced every column, so live samples that arrived between the read's snapshot and its
  apply were lost; the realtime poll never delivers a sample twice.
• At a coarse layer a live sample folds into the last column. When the coarse read did not apply, the
  columns were still Raw, the whole coarse phase folded into one Raw column, and a zoom back to the same
  width found the Raw fetch covering the window and read nothing.
• The fresh tail clamped four point spacings back from the requested right edge, which the prefetch margin
  puts one window past the visible end, so a coarse read never reached the newest raw rows.

Change

• Live tail across an apply. EnvelopeLine.ReplaceColumns(columns, requestedEndX) keeps the columns
  appended past the last history X that lie after the new last column and at or before the requested edge,
  then trims to the cap. A pan into the past drops the follow tail, so the band just read keeps the buffer.
  ClearColumns is gone: a pen absent from a result keeps its live tail. CurrentValue reads the merged
  columns.
• Fold over Raw. A fold while the fetched range is Raw clears it, so the return to Raw reads Raw. A fold
  that meets a gap column opens a column past it instead of dropping the sample and freezing the reading.
• Fresh tail. The clamp is the earlier of the requested edge and the present; PostgresDataProvider
  takes an injected TimeProvider (TryAddSingleton(TimeProvider.System)).
• History read log. Every read logs one Debug entry (layer, range, column target, pen count, outcome,
  envelope count, duration) beside the realtime poll's per-tick entry.
• Docs in docs/architecture/charting.md and data-integration.md; the plan moves to
  docs/plans/completed/.

Verification

• Acceptance tests, each red with its fix reverted and green at HEAD:
  AReturnToRawAfterAHeldCoarseReadReadsRaw, AnApplyKeepsTheLiveColumnsPastItsSnapshot,
  TheRangeTheChartAsksForReachesTheRawLayersNewest (09:04:50 UTC against 09:04:00),
  AValueAfterACoarseEnvelopeEndingOnAGapTakesTheReading; AHistoryReadLogsItsRequestOutcomeAndDuration
  pins the log.
• dotnet test SemiPlot.slnx: unit 1822/1822, integration 147/147. dotnet format --verify-no-changes
  clean.
• Manual test on the demo stand in follow mode: no straight segment at the live edge.

Per-task commits before collapse

13a15e2 docs(plans): state the gap fold rule plainly
9309ad7 docs(plans): record the live tail gaps run
1241a42 fix(chart): resume the fold after a gap column
36b95f3 refactor(chart): trim comments the docs already hold
d8f34d8 refactor(chart): address smells review findings
38dfc5b fix(chart): address review findings
a5e16f8 docs(plans): close the live tail gaps tasks
eb857c2 docs(plans): record the live tail gaps checks
732789b fix(postgres): clamp the fresh tail at the present
6809d62 fix(chart): keep the live tail across a history apply
f3e1077 fix(chart): read Raw again after a fold over Raw
29b0e7b feat(postgres): log every history read
0d39406 docs(plans): add live tail gaps plan