Add NonIdentityPoint::new_from_constant

[ConstanceBeguier]

This PR adds NonIdentityPoint::new_from_constant to enable creating non-identity points that are properly pinned to constants, which is required by the Orchard ZSA circuit (Orchard PR).

More precisely, in the Orchard ZSA circuit, q_init_zec / q_init_zsa must be constrained to fixed constants, as they define the initial point Q of the Sinsemilla hash. If constructed via NonIdentityPoint::new, they remain unconstrained witnesses, allowing a prover to inject an arbitrary on-curve point and break commitment soundness.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 68.75000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.36%. Comparing base (7e76b1c) to head (d44d277).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
halo2_gadgets/src/ecc/chip/witness_point.rs	37.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #887      +/-   ##
==========================================
+ Coverage   83.31%   83.36%   +0.05%     
==========================================
  Files         105      105              
  Lines       13054    13048       -6     
==========================================
+ Hits        10876    10878       +2     
+ Misses       2178     2170       -8     

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[daira]

This changes because of the call to super::chip::witness_point::tests::test_witness_non_id in MyEccCircuit::synthesize. But we need to ensure circuit stability, so actually MyEccCircuit should have a test_zsa_additions flag that is set to false in order to check stability of the existing fixtures (both _insecure and _fixed after halo2_gadgets 0.5.0), and to true for the circuit variant that tests non_constant_point_id as well (there we only need the "fixed" variant).

[daira]

Blocking requests:

• rebase on top of halo2_gadgets 0.5.0 when that merges;
• add a flag to MyEccCircuit so that the existing stability checks (test_ecc_chip_*_against_stored_circuit) continue to pass without changing the *.bin,*.rdata fixture files; and add new stability checks with new fixture files (only the circuit_version = AnchoredBase variant needed).

Otherwise this looks good, we'll merge it and release as halo2_gadgets 0.5.1.