Optimize StateVectorLQubitManaged CTOR

[vincentmr]

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---

Context:
Profiling a simple script similar to

from pennylane_lightning.lightning_qubit_ops import StateVectorC128
import time

for nq in range(2, 32, 2):
    t0 = time.time()
    dt = 0.0
    niter = 0
    while niter < 5 or dt < 1.0:
        niter += 1
        sv = StateVectorC128(nq)
        dt = time.time() - t0
    print(f"{nq}, {dt / niter:1.3e}")

it appears some time is wasted in a duplicate memset call when invoking the StateVectorC128 constructor.

Description of the Change:
Directly set the 0th element to 1.0.

Benefits:
Faster StateVectorC128 initialization.

Possible Drawbacks:

Related GitHub Issues:
[sc-69660]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.32%. Comparing base (de7beb7) to head (9dc37ff).
Report is 91 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (de7beb7) and HEAD (9dc37ff). Click for more details.

HEAD has 5 uploads less than BASE

Flag	BASE (de7beb7)	HEAD (9dc37ff)
	12	7

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #811      +/-   ##
==========================================
- Coverage   98.60%   92.32%   -6.28%     
==========================================
  Files         114       73      -41     
  Lines       17658    11172    -6486     
==========================================
- Hits        17412    10315    -7097     
- Misses        246      857     +611     

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[maliasadi]

🚀

[LuisAlfredoNu]

Nice Job. brief change with good results