estimateProjectedBranchTokens function

int estimateProjectedBranchTokens(List<SessionRecord> branch)

Projection-aware variant of estimateSessionBranchTokens (issue #503 round 3b): counts what Session.buildContextMessages would ACTUALLY project, not the raw branch. Three adjustments over the raw tally:

1. Classic transform — everything before the last CompactionRecord.firstKeptEntryId is dropped (the summary replaces it).
2. Structured hiding — a record id in any HiddenRangeRecord on the branch renders as a one-line marker, not its full content.
3. Checkpoints — records covered by a CompactCheckpointRecord (including flattened inner checkpoints, D4) count zero; the checkpoint's own text counts once.

On a structured-compaction marathon the raw estimate overshoots the real projected context by an order of magnitude (hundreds of hidden tool results), which tripped the resume walk's token budget far too early and resumed with a badly under-filled window.

Implementation

int estimateProjectedBranchTokens(List<SessionRecord> branch) {
  // Classic transform: the last compaction drops its prefix (mirrors
  // SessionTree._applyCompactionTransform: kept = [compaction] +
  // path[firstKept..compactionIndex) + path(compactionIndex..]). The
  // summary record itself survives and REPLACES the dropped span.
  var kept = branch;
  for (var i = branch.length - 1; i >= 0; i--) {
    final record = branch[i];
    if (record is CompactionRecord) {
      final cut = branch.indexWhere((r) => r.id == record.firstKeptEntryId);
      final foundKept = cut >= 0 && cut < i;
      kept = [
        record,
        if (foundKept) ...branch.sublist(cut, i),
        ...branch.sublist(i + 1),
      ];
      break;
    }
  }
  // Structured state over the kept region (mirrors buildStructuredViewState;
  // re-implemented here because projection.dart already imports this
  // library — importing back would be circular).
  final hidden = <String>{};
  final covered = <String>{};
  for (final record in kept) {
    switch (record) {
      case HiddenRangeRecord(:final recordIds):
        hidden.addAll(recordIds);
      case CompactCheckpointRecord(
        :final coversRecordIds,
        :final firstRecordId,
        :final lastRecordId,
      ):
        covered
          ..addAll(coversRecordIds)
          ..add(firstRecordId)
          ..add(lastRecordId);
      default:
        break;
    }
  }
  var tokens = 0;
  for (final record in kept) {
    if (covered.contains(record.id)) continue;
    if (hidden.contains(record.id)) {
      tokens += _hiddenMarkerTokens;
      continue;
    }
    tokens += switch (record) {
      MessageRecord(:final message) => estimateTokens(message),
      CustomMessageRecord(:final content, :final timestamp) => estimateTokens(
        UserMessage(content: content, timestamp: timestamp),
      ),
      CompactionRecord(:final summary) =>
        (summary.length / _charsPerToken).ceil(),
      BranchSummaryRecord(:final summary) =>
        (summary.length / _charsPerToken).ceil(),
      CompactCheckpointRecord(:final text) =>
        (text.length / _charsPerToken).ceil(),
      _ => 0,
    };
  }
  return tokens;
}