CSV and TSV summary exports

Use --summary=ext --format=csv for a spreadsheet-ready extension summary, or select tsv for
xff's tab-separated, backslash-escaped format. Both contain one header for the entire export.
Repeated summary requests and multiple scopes append data rows under that same header.
--no-header removes that header once. There are no headings, legends, blank separators, human
size suffixes, or thousands separators in the export.

xff project-a project-b --summary=ext --summary=type --summary-scope=root --format=csv
xff --compare=summary LEFT RIGHT --summary=ext --summary-scope=diff --format=csv

The second command includes comparison-result rows followed by extension rows with left-only,
right-only, and different column groups. Their counts and byte denominators retain the normal
summary scope semantics. Each category counts pairs once; its bytes
include both sides. Side-total scopes count and size only their own side.

Columns

Columns	Meaning
`record`	Always `summary`
`request`	Zero-based summary request identity, preserving repeated requests
`summary`, `template`	Canonical grouping and its field template, when applicable
`scope`	`all`, `root`, `compare`, or the canonical selected comparison scopes
`root`, `left_root`, `right_root`	Root identities where applicable; otherwise empty
`type`	Entry type for comparison-result rows; otherwise empty
`group`, `is_total`	Actual grouping key and explicit boolean total-row marker
`count`, `count_percent`, `bytes`, `size_percent`	Metrics for ordinary or comparison-result rows
`SCOPE.count`, `SCOPE.count_percent`, `SCOPE.bytes`, `SCOPE.size_percent`	Metrics for each selected comparison column group

Scope-prefixed columns follow the resolved selector order and appear when ordinary comparison
summaries need them. Every row has the same columns; inapplicable cells are empty. A missing
comparison cell is empty, distinct from a present metric of 0. A summary without a byte dimension,
such as a value-stream extraction, leaves byte cells empty. is_total=true distinguishes the
synthetic total from a real group whose name is total or empty.

Counts and bytes are exact base-ten integers. Percentages use --summary-precision; --human does
not change these numeric columns. --top limits group rows but leaves totals and denominators
covering the complete population.

Escaping and composition

CSV uses the existing listing encoder: commas, quotes, CR, and LF trigger quoting, and embedded
quotes are doubled. Use a CSV parser rather than splitting lines or commas: a quoted cell can
contain a newline.

TSV uses the existing listing convention, not CSV quoting: backslashes, tabs, CR, and LF are
encoded as \\, \t, \r, and \n. Split tab-separated records, then decode those escapes
once in each cell. Quotes are ordinary characters. UTF-8 content remains UTF-8.

CSV/TSV summary exports cannot include histograms or action output. Select comparison summaries
with --compare=summary, or suppress per-path records using --compare-select=none. Conflicting
output producers and applicable dry-run previews are rejected before traversal or actions.
Non-output predicates, collections, and captured command results can still feed summaries.
--columns remains a listing control and cannot replace this schema.