feat: matured TaoFlow EMA to reduce capital recycling attacks

[igoraxz]

Problem

The user-flow EMA that drives emission allocation reacts symmetrically to inflows and outflows. That symmetry lets a participant temporarily inflate a subnet's emission share with capital that is later withdrawn, rather than rewarding durable demand.

Change

Add a second EMA on top of the existing user-flow EMA, and use the lower of the two as the signal:

raw     = flow EMA of (buys - sells)     [existing]
slow    = flow EMA of raw                [new]
matured = min(raw, slow)                 [used in the share calc instead of raw]

Because slow lags raw, the clamp is asymmetric:

• Inflow spike -> raw jumps above slow -> matured = slow: credit accrues slowly.
• Outflow -> raw drops below slow -> matured = raw: the decrease applies immediately.
• Steady genuine flow -> raw ~= slow -> matured ~= raw: no penalty for real demand.

So transient inflows are credited only at the slow EMA's pace, while withdrawals take effect immediately. Durable, sustained inflow still converges to the full signal.

Design notes

• No new parameter -- slow reuses FlowEmaSmoothingFactor. A sweep of the maturity half-life against both manipulation resistance and honest-subnet bootstrap time put the optimum at the same half-life as the main flow EMA: it is the knee of the trade-off (shorter weakens the clamp, longer barely improves resistance while slowing new-subnet onboarding). Equal factors also make slow a clean double-EMA of the flow.
• SubnetEmaSlowTaoFlow stores the unclamped EMA(raw), not min(raw, slow) -- the clamp is applied only at read time, so the slow EMA keeps tracking the true long-run signal and can recover.
• First access seeds slow = raw so existing subnets see no emission cliff at deployment.
• Protocol EMA is not matured -- only user demand is delayed.

Test plan

• Slow EMA persists and smooths; seeds at raw on first access (no cliff)
• Inflow spike -> matured < raw; outflow -> matured = raw; steady flow -> matured = raw
• SubnetEmaSlowTaoFlow cleaned up on deregistration

Builds on #2675 (normalized protocol cost), already merged to devnet-ready.