feat: tune default min gas price

[danbaruka]

Summary

Drops the default --minimum-gas-prices from 100000usaf (set in #25) to 0.05usaf so that day-one transaction fees on Safrochain stay in the cent range instead of costing several thousand SAF per send.

Why this is needed

The current default of 100000usaf per gas unit was off by roughly 6 orders of magnitude. A standard MsgSend at the typical ~80 000 gas would cost:

80 000 gas × 100 000 usaf/gas = 8 000 000 000 usaf = 8 000 SAF

That is more than any community validator's self-stake. In practice it would either:

• block all economic activity on the chain, or
• force every operator to override the default in app.toml, which silently splinters the fee market.

The default needs to be a sane floor that operators could override upward, not a number they must override downward.

Proposed value: 0.05usaf per gas

Calibrated against typical Cosmos SDK gas costs:

Operation	Typical gas	Fee at 0.05 usaf/gas	Human-readable
MsgSend	80 000	4 000 usaf	~0.004 SAF
MsgDelegate	200 000	10 000 usaf	~0.010 SAF
IBC MsgTransfer	200 000	10 000 usaf	~0.010 SAF
CosmWasm contract call	1 000 000	50 000 usaf	~0.050 SAF

This keeps fees in the single-cent range for end users while still producing meaningful validator fee revenue (roughly 10× higher per-tx than the 0.005usaf value I evaluated earlier).

Why not 0?

A non-zero default protects the chain from trivial spam at the mempool layer before governance has the chance to set a globalfee floor. 0.05usaf is small enough not to harm users and large enough that flooding the chain costs real value.

Why this is the right place to set it

DefaultMinGasPrices in cmd/safrochaind/cmd/root.go is the value that gets written into a fresh app.toml on safrochaind init. Operators can still override via app.toml minimum-gas-prices or the --minimum-gas-prices CLI flag — this change only sets the sane default for new installs.

Changes

• cmd/safrochaind/cmd/root.go: DefaultMinGasPrices "100000usaf" → "0.05usaf" (1 line)

Out of scope

• On-chain x/globalfee minimum (governed by params, not this default).
• Validator-level overrides via existing app.toml. Existing validators are unaffected; only freshly initialised nodes pick up the new default.

Test plan

• make install and confirm safrochaind init writes minimum-gas-prices = "0.05usaf" in ~/.safrochain/config/app.toml
• Start a single-node devnet and submit a MsgSend with --fees 4000usaf — accepts.
• Submit the same MsgSend with --fees 100usaf — rejected with "insufficient fees".
• Existing nodes upgrading do not see their app.toml overwritten (default is only applied on init).

Release notes

Lowered the default minimum-gas-prices from 100000usaf to 0.05usaf. Existing nodes are unaffected; new safrochaind init runs will pick up the lower default. Operators who want a higher floor can still override in app.toml.