phase 1 review fixes: DST notation + cross-link expectations note

Issues found during review:

1. Sessions DST notation was misleading — the '/ XX:00 (non-DST)'
   alternates implied a year-round shift, but in reality NY DST and
   UK BST mostly track each other; only ~3 weeks/year are misaligned.
   Replaced with canonical NY-time anchors and a pointer to
   04-time-cycles/dst-handling.md (Phase 2) for the actual nuance.
   Affected: session-overview, asia-session, london-session,
   london-close, session-overlaps.

2. Forward cross-links to Phase 2-7 files don't yet resolve. This is
   expected per build plan; documented in README.md so readers know
   it's intentional, not a bug. Phase 8 is the link-check sweep.

JSON + structure validation re-run: all 35 files clean.