emrg: fix flaky session ID uniqueness test — 16-bit → 32-bit entropy

[argszero]

Summary

Fixes a flaky CI failure on master push run 31145421676:

tests/test_session.py::TestGenerateSessionId::test_generates_unique_ids FAILED
E       AssertionError: assert 9 == 10

Root cause

generate_session_id() used secrets.token_hex(2)[:4] — only 16 bits of entropy (65536 possibilities) for the per-minute suffix. The test generates 10 IDs in the same minute, so the birthday paradox gives a ~0.07% collision chance per run. It happened on the master push after #538 merged (PR CI was green because it sampled a different random draw).

Fix

Suffix now uses secrets.token_hex(4) (4 bytes = 8 hex chars = 32 bits). Collision probability drops ~65500x to ~1e-8 per run. Format remains s_YYMMDD_HHMM_xxxxxxxx; existing format assertions (len >= 14, parts >= 4, YYMMDD/HHMM lengths) all still pass.

Verification

• 5 consecutive runs of TestGenerateSessionId — all pass
• Full suite: 502 passed + import OK + CLI OK

[argszero]

✅ LGTM — cycle (1/3). Self-review: 16-bit → 32-bit suffix entropy fixes the flaky 10-sample uniqueness test (birthday-paradox collision ~0.07% → ~1e-8 per run); format assertions unaffected; 502 tests pass + 5x repeat of TestGenerateSessionId green.

[argszero]

✅ LGTM — cycle (1/3). Self-review: 16-bit → 32-bit suffix entropy fixes the flaky 10-sample uniqueness test (birthday-paradox collision ~0.07% → ~1e-8 per run); format assertions unaffected; 502 tests pass + 5x repeat of TestGenerateSessionId green.

[argszero]

✅ LGTM — cycle (2/3). Re-verified: token_hex(2)[:4] → token_hex(4) (16→32-bit entropy); collision probability ~0.07% → ~1e-8 per run (~65500x); format assertions (s_YYMMDD_HHMM_xxxxxxxx) unaffected; 502 tests pass + 5x TestGenerateSessionId repeat green. CI green (31145767064).

[argszero]

✅ LGTM — cycle (3/3). Final re-verification: token_hex(4) gives 32-bit entropy (collision ~0.07% → ~1e-8 per run); format s_YYMMDD_HHMM_xxxxxxxx preserved; flake empirically confirmed via rerun of 31145421676 (same commit, one red one green); 502 tests + 5x repeat green. CI green (31145767064). 3 consecutive ✅ from different cycles — mergeable.