fix(monitor): coordinated sampling allocates on per-sid (agg-id) freq f_i, not total rate

[zzylol]

Problem

coordinator.rs::allocate_p() passed the edge rate vector as both the rate and the freq argument to allocate_sample_rates():

let p_vec = allocate_sample_rates(&rates, &rates, var_budget);

Since p_i = clamp(√λ·√(f_i/rate_i), 0, 1) and f_i == rate_i, every √(f_i/rate_i) = 1 → the allocation collapses to a uniform p. All edge differentiation was left to the ε-floor (which depends on rate only). So the headline p_i ∝ √(f_i/rate_i) coordinated-sampling claim was not actually exercised — the comment even said "freqs proxy = rates (no per-key split available at the coordinator)."

Fix

The MonitorReport already carries known_value — the monitored functional's per-edge value this epoch (the cms_point key frequency, or the edge's contribution to the monitored sum), which is the correct f_i and is distinct from the edge's total update rate. Feed it as the freq vector:

let freqs: Vec<f64> = ids.iter().map(|id| self.edges[*id].known_value).collect();
let p_vec = allocate_sample_rates(&rates, &freqs, var_budget);

Now an edge carrying more of the monitored key at equal total rate keeps a higher p; an edge with no monitored-key mass (known_value=0) falls back to p=1.

Tests

• New per_key_freq_differentiates_at_equal_rate: equal rate, different known_value ⇒ different p, both above the equal rate-floor (fails on pre-fix code).
• Existing skewed_rates_yield_differentiated_sample_p_above_floor still passes.
• 13/13 monitor::coordinator tests green.

Found while wiring Fig 9 (coordinated sampling) on the 8-node eval cluster.

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