Within the Bitcoin ecosystem where are wallet addresses stored/reserved?
When you create an address, it is stored in your wallet. It is not recorded on the blockchain until someone executes a transaction that includes the address.

It is not necessary to reserve your address since it is statistically impossible that someone else will generate the same address (with some caveats, see below). Here is what happens when you create a new address in your wallet:


Your private key (for spending funds) is derived from a random number between 1 and 115792089237316195423570985008687907852837564279074904382605163141518161494337. Assuming the number was truly random, you don't need to worry about someone guessing this number since the number of possibilities is so ridiculously large.
The matching public key (for receiving funds) is derived from the private key in a way that is irreversible, and is used to generate the address where you can receive payments. There are 2^160 possible addresses (1461501637330902918203684832716283019655932542976).
The private and public keys are saved together as a "pair" in your wallet.