Add optional funding target to escrow (#144)

[laurentketterle-hub]

Closes #144.

Adds an optional target: Option<i128> to fund(), stored on the Escrow
record and read-only thereafter. It is informational only — it does not block
contribute from pushing amount past it and does not change
release/refund behavior — but it gives sponsor-facing UI an on-chain
"raised X of target Y" number instead of one tracked off-chain that can drift
from on-chain truth.

Changes

• contracts/escrow/src/types.rs: add target: Option<i128> to Escrow.
• contracts/escrow/src/lib.rs: accept and store target in fund(); document it.
• contracts/escrow/src/test.rs: pass None at every existing fund call site,
  and add test_fund_stores_optional_target_and_contribute_past_it covering:
  • Some(target) round-trips through get_escrow,
  • contribute past the target grows amount but never mutates target,
  • None is the default.
• README.md + docs/escrow-crowdfunding-design.md: reflect the new field.

Notes

• No setter is added: target is set once at fund() and never mutated, so it
  can't drift from what the original sponsor declared.
• Because target is informational, no new error variants are needed and the
  payout path (release/refund) is untouched.

Verification

• cargo fmt --check — clean
• cargo clippy -p mergefi-escrow --all-targets -- -D warnings — clean
• cargo test -p mergefi-escrow — 37 passed, 0 failed

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