notations: family selection — FGCA, FGA, Goals, Activities

[transitrix]

Summary

• Replace the placeholder "Picking between..." paragraph in notations/README.md with a fuller Family selection section.
• New section answers all four AC items in one place: layer composition (F/G/C/A coverage per notation), a situation-to-notation selection matrix, and the form rule (nested for tree-shaped FGA/Goals, flat for DAG-shaped FGCA/Activities).
• Specs 02-fgca, 03-fga, 04-goals, 07-activities each gain a one-line cross-link to the new section.

Closes vkgeorgia/strategy#17.

Where the matrix lives — coordinated with #10

Per the agreement in #4 (closing strategy#10), the family selection matrix lands in notations/README.md rather than its own file. The four specs link to it; the README owns it. No duplication.

Form rule

Documented the decision recorded 2026-05-20 alongside the FGCA schema (#7): if a child element can have multiple parents in the semantic graph, the notation uses the flat form. Examples: FGCA (one Change → many Goals; one Activity → many Changes) and the Activities network (predecessor fan-out) → flat. FGA and the Goals tree (single parent per child) → nested.

Scope guard

Per the issue refinement, this PR touches only the README family-selection section and adds the cross-link line in each of the four specs. No spec rewrites beyond that line; no example edits; no audit items.

Cross-link placement

• 02-fgca.md on main has no References list today, so the cross-link lands in a new "See also" section at the end. When notations/02-fgca: add flat-form schema and rewrite example #7 merges (which introduces FGCA's References list), the cross-link will sit alongside the other references. The two locations don't conflict but a future cleanup could fold them together.
• 03-fga.md, 04-goals.md, 07-activities.md add the line to their existing References list.

Test plan

• notations/README.md carries the Family selection section with the three subsections (layer composition table, selection matrix, form rule).
• Each of 02-fgca, 03-fga, 04-goals, 07-activities contains exactly one new line linking to the README section (grep "Family selection" notations/*.md shows them all).
• No example files or other spec sections touched.
• No links to the private vkgeorgia/strategy repo in any shipped file.

🤖 Generated with Claude Code

[transitrix]

Superseded by #11.

This branch (task/17-family-selection) forked before the CONTRACT, FGCA-schema, IDs-appendix, and status-vocabulary PRs merged, so it can no longer merge cleanly into main. #11 carries the identical change rebased onto current main — the diff is verified as exactly the intended set (README "Family selection" section + one cross-link line in 02-fgca / 03-fga / 04-goals / 07-activities) and merges without conflict.

Closing in favour of #11.